What are the benefits to using synthetic oil? I hear that Cadillacs are notorious for burning oil. Will synthetic fix it? Will the motor last longer?

Unless the motor is specified for synthetic oil (like the rear wheel drive VVT Northstar engines) there is absolutely no reason to use it. No advantages. No the motor will not last longer. No you cannot extend the drain interval. No it will not help oil consumption...in fact, it has been shown to aggravate it in some cases.

This topic has been covered many times. If you go to the search feature in the forum tool bar and type "synthetic" into the drop down box and search the archives you may save us another flaming arguement over synthetic oil....LOL LOL Always a hot topic.

YES! Please save us from the ongoing oil debate! LOL!

Synthetic oil does have BIG advantages. You can run it for extended drain intervals, if, the oil is designed and formulated to do so. I use Amsoil in my diesel truck. It does not have to be changed, ever, but you do have to keep it clean. Good synthetic oil does not break down, it just gets dirty like everything else. If you run it for extended intervals, it is a good idea to send off a sample every 20k or so to make sure it is not contaminated.

As far as using it in your caddy, probably not a good idea. It has been known to find leaks and increase oil consumption. I say "find" leaks because I dont believe that it causes them, it just washes away the petroleum deposits that are preventing leaks in your engine. The end result is the same.....the stuff on the inside, winds up on the outside!

Agreed, synthetic oil does not break down. For proof, I've been changing oil on my mothers '89 Olds Cutlass Supreme 2.8L since about 5 years ago starting at 38k miles with Mobil synthetic (mom doesn't keep very good track of mileage so I resorted to trying synthetic). The interval was annually, regardless of the mileage. I am happy to say that it just turned over 100k this year (long drive to work and back for her) and the motor doesn't make a tick of any sort. No oil consumption yet either. Now, I am no mechanic, but I doubt an engine would be in the condition of this 2.8 if the oil were changed at this interval using some regular conventional oils. I have no proof that synthetic fails at what it is believed it can do, therefore I have no reason to stop using it. I'm not saying anyones right, I don't think anybody really knows. But, until it fails me, I am loyal to it in this situation.

sorry, make that four years ago

But, until it fails me, I am loyal to it in this situation

Thats all that matters! Great to hear an honest synthetic oil success story.

Here we go again.....LOL


There are many reasons to change oil. Depletion of the antiwear additives (the ZDP), oxidation of the oil due to heat, contamination of the oil with water/gasoline/byproducts of combustion, contamination of the oil with pentane insoluables such as dust/dirt/carbon, acid buildup, etc.

Synthetic oil is capable of withstanding extended intervals of operating temperatures above 300 F....but....there are no passenger cars or trucks that are designed to allow the oil to get that hot in normal use. So...the single advantage of synthetic is never exploited in most all passenger cars and trucks and therefore is pointless.

The high temperatures oxidize the oil. If that was the ONLY reason to change the oil then synthetic would allow extended drain intervals. As itemized above, there are other, more pressing reasons, to change the oil. Synthetics do not have any more anti wear compound concentrations, no more acid resistence, no more resistence to contaminate buildup, etc.... Since those items account for 99% of the oil change requirements and dictate the oil change interval the synthetics do not offer any advantage.

Synthetics do NOT allow a longer change interval. Notice that Mobil 1, the premier synthetic, does NOT mention longer drain intervals.....

Amsoil maintains their capability of longer change intervals purely based on their performance on the sequence 3E oil oxidation tests at high temps. True, their synthetic will not oxidize as fast at the elevated temps of that test. But, that test only quantifies A SMALL PORTION of the oils overall performance. Amsoil does not have any more anti-wear compounds. Amsoil is just as susceptable to water and fuel contamination on cold starts and short trip driving. Amsoil has no more anti-acid compounds. etc...etc... Amsoil's claims of longer drain intervals are simply wrong and the reason that non of the automakers or engine manufacturers will deal with them or recognize their products. There is just no scientific basis for their claims. They ignore 99 percent of the reasons for oil changes and base their claim on one selective criteria that means little in the real world.

Conventional mineral oils will protect the engine just as well and will more than meet the requirements of most engines.

Some engines are mandated for synthetics such as the VVT RWD Northstars, the Corvette engines, the DOHC CTS V6 with VVT, etc... Those engines are designed for potential track use that can drive the oil temps above 300 for short periods of time so the use of synthetic is desireable to alleviate the need for an auxilary oil cooler. In addition, the VVT systems use the oil as a hydraulic fluid so the more linear viscosity improves the VVT control at very low and very high temps...so the VVT system is designed to use the synthetic. The lube system could care less about this piece of the puzzle.

Don't get me wrong. The synthetics, and Mobil 1 in particular, are excellent products and do offer some advantages in very cold ambient cold starting/cranking speeds (if you are seeing -40 and below regularily) and vey hot operation...like track days. In between, where 99.9 percent of the population operates, they are completely unnecessary. They won't hurt anything (unless you try extending the drain intervals) but they won't help anything either.

I've been a long time regular motor oil guy and recently changed to Mobile1. I've only seen two big improvements:

1. Car starts up a hell lot better in this crazy cold northeastern weather. It started up as if it's 30-40 F outside when it's actually -20 F.

2. My deville 99 uses less oil when I drive 300 miles non-stop at 80-90mph. It used to consume 3/4 of a QT of regular motor oil, now just 1/4 QT of mobile1.


I certainly do like the idea of having Mobile 1 synthetic ATF in my transmission though. At 50k, my transmission was already starting to hard shift at 1-2 and 3-4. Dropped the pan, emptied out that hidden sump (thanks bbobynski for that tip), and dumped in 8 QTs of M1 atf. It took 300 miles of driving to get the transmission completely smooth again. Now, it runs like it was brand new.

Personally, I don't think it was worth to use synthetic motor oil, not enough benefits to offset the $45 cost (8.5qt + filter), but I think it's worth to use synthetic transmission oil. $48 for 8qt of mobil1 atf, change every 2yrs.

thanks for the help and sorry about starting a discussion about something already discussed. still new to this forum stuff. :banghead:

thanks for the help and sorry about starting a discussion about something already discussed. still new to this forum stuff. :banghead:


Yea....it's all YOUR fault....LOL LOL LOL just kidding. Welcome aboard.

I agree totally with Bbob. I know how much mis-information there is out there on this particular subject, and have quite often been on the unpopular side of debates on the subject. As was pointed out, other than having a wider operating temperature spectrum, the only thing synthetics do is empty your wallet that much faster.

Here is why I use it.

My truck holds 11 quarts. 11 qts of dino oil is about $22.00

Oil filter, $8.50

Changing oil and filter every 4,000 miles, in 12,000 miles I spend $91.50

Synthetic amsoil at $3.65 a qt x 11, $40.15

2 oil filters ( for 2 changes in 12,000 miles ) $17.00

2 extra qts to add at filter changes, $7.30

My cost to change only the filter and add a qt every 4,000 miles, and change the oil every 12,000 miles is only $64.45

I believe the synthetic is better, and for me, it is cheaper.

about 12k mile oil changes :

Follow the oil life monitor recommendations. It is the only accurate way to know when to change your oil as it takes all the things that require an oil change into account including engine revolutions, engine load, engine operating temps and driving schedule, etc. This is important as it constantly monitors YOUR driving schedule and updates the oil life and change interval accordingly. It is entirely possible to go 12000 miles on an oil change if the driving is done on long trips. Short trips and winter driving will see much quicker change intervals on the oil life monitor.

Understand that the engine oils of today are significantly better than oils of yesteryear and they can easily accomodate the extended change intervals under the correct conditions. Also, modern engines such as the LS6 are designed to not deplete the oil significantly. The PCV systems are much more efficient at cleaning the crankcase, the major rubbing elements that required high antiwear protection are eliminated and the engine is designed to be relatively insensitive to lubricant quality. Things like spur gear oil pumps that sheared the oil down in viscosity grades , rubbing element tappets and rocker arms and distributor/oil pump drive gears are eliminated. The gerotor oil pumps are easy on the oil. Roller lifters and roller rocker arms do not require excessive lubrication and do not deplete the antiwear additives in the oil nearly as fast. That is why the oil change intervals on later model engines are going higher and higher in miles. The engines are designed for the longer change intervals and will accomodate them fine. This is NOT to imply that the longer change intervals are retroactive to older engines....follow the manufacturers recommendation.

This is discussed to great length in the post on "synthetic oils" in the general discussion area under "non-specific model discussions..." read up on it here http://www.cadillacforums.com/forums/showthread.php?t=10683

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on a side note , we got more than 12k out of mobil 1 delvac on the 1995 5.9 cummins in the ram pickups with no ill effects , like you do we changed the filters after doing an oil analisisys .....only when the oil turned up with high acidity levels did we go ahead and change the oil ....later found out that acidity levels alone werent good enough , who knows how the anti wear packages held up over all that time ......its not like we babied those engines either ....they towed 10-15,000 lbs and had a max speed of 35 mph from the wickedly short rear gear in them...

Here is why I use it.

My truck holds 11 quarts. 11 qts of dino oil is about $22.00

Oil filter, $8.50

Changing oil and filter every 4,000 miles, in 12,000 miles I spend $91.50

Synthetic amsoil at $3.65 a qt x 11, $40.15

2 oil filters ( for 2 changes in 12,000 miles ) $17.00

2 extra qts to add at filter changes, $7.30

My cost to change only the filter and add a qt every 4,000 miles, and change the oil every 12,000 miles is only $64.45

I believe the synthetic is better, and for me, it is cheaper.


You must be an Amsoil dealer. Where do you get it for $3.65 a quart? Even the cheap synthetic 10W-30 is about $5.20 a quart, and the regular stuff is $5.95. What kind of filters do you use?

You must be an Amsoil dealer. Where do you get it for $3.65 a quart? Even the cheap synthetic 10W-30 is about $5.20 a quart, and the regular stuff is $5.95. What kind of filters do you use?


That's the other thing about Amsoil...if their stuff was so good you would expect to see it on the shelves of every store...not sold thru some pyramid scheme "dealer network...".....LOL LOL

Anyone that thinks that Amsoil knows something that the major oil companies do not is in for a rude awakening.

I use Mobil 1 synthetic in the '93 DeVille and '79 DeVille, the Olds gets regular Mobil Drive Clean.

My father is an AMSoil dealer and says they are the best makers of synthetic oil, and I tell him I am going to stick with Mobil 1, as I perfer it and from things I have heard, it is actually better...

Yesterday when I was there, we talked about it for a minute or 2, and he said "look, on the side of the box they have all the tests where it outperforms other oils... even Mobil 1" at that time I said forget it...

He used to use Castrol SynTec, which isn't very good... not even a "real" synthetic.... anyway I can get him out of the "AMSoil claw of death"?

I have my own reasons for using the Mobil 1.... I don't like to get into the arguments anymore, so I say that A) it makes me feel better and B) I don't mind spending the extra money to do an oil change. I do it myself, so it off-sets anything I would pay to have someone else do it.

the oil on the '93 gets changed according tot he life monitor, I wait for it to get to 0%, it is about every 5k miles. I change the oil on the Olds every 4-5k miles depending on use.

Here is a question though. how often should I change the oil in the '79 DeVille? it dosn't get driven much (in comparison to the Olds and '93)... so say 3,000-5,000 miles/year. Should I just change the oil once/year, or should I change it more often because the engine sits sometimes with the "old" oil in it etc... the last oil change (changed when I first got the car) went from May '04 - Dec. '04, but there was only 2,000 miles on it, but I changed it anyway because I "thought" the car was going to be sitting all winter.

Amsoil was the first synthetic oil. Amsoil makes the best extended drain diesel 15W40 oil. And yes, I am an Amsoil dealer. Mobil 1 is great oil, for cars. For that matter, I see no reason why any one synthetic would be any better than any other. The reason I use Amsoil, they make pretty much everything synthetic. From oil, to gear lube, to grease, to a wet clutch compatible formula for my big v-twin motorcycle. I can get it all in one place.