: Synthetic Oil - How often to replace?

11-26-03, 11:55 PM
Well, I'm at about 3,000 miles since my last oil change. I'm using 20w50 Synthetic.. Do I need an oil change? Or is it okay to wait until 5,000 miles? Most of my trips are short - around 15 minutes long...

11-27-03, 12:26 AM
Replace it Sal...... Dont wait until 5k, esp since you do short trips..... I guess it comes down to how long you wanna keep it! The longer you wait, the more sludge and varnish can accumulate!

Its best to do it every 90 days (or 4000 miles, whichever comes first)..... Oil can go bad just sitting in the car not being used!!!!!

Even if you do 90 day oil changes, that means you only need to have it done 4 times a year.... I think you can cover that.... no sense in waiting......

11-27-03, 12:46 AM
Synthetic breaks down differently then normal oil and can go longer. We use regular oil so I don't have any advice on it. But I did find this:


Night Wolf
11-27-03, 01:21 AM
I run 10W-30 Mobil 1 and go 5k miles between oil changes... I use AC Delco filter too..... it can definitly do it.....

this artical right here pretty much proved to me that 5k miles is nothing at all and 3k miles is overrated....

....that and the fact that Richie (mothers boyfriend) has been using dino oil in his '87 Brougham since he got it.... with 75k miles on it, and it now has 180k miles, no major engine work (only major thigns were carb rebuilt when he first got the car and the tranny rebuilt last year) he even hauls a trailer with the car.... changes the oil every 5k miles.....

this is a good read:


11-27-03, 02:04 AM
For the first time in this Caddy, I am using Mobil 1 10w30. I have put about 1,100 kms, and the oil-life monitor says 78% good still. No leaks, no level drops accused with syn.

Anyway, with my Grand Marquis I ran the Mobil, and it got darker, quicker. I have a feeling that it is because the previous owner didnot change it as often, so I had to change it more just based on appearance of the oil alone. I remember Wes mentioning that syn. is thinner or something, so it may appear darker (was that you Wes), so maybe it does not matter if this happens, but the anti-freezing factor is THE reason I wanted to use it in the cold weather. (1st winter) Yes, syn does not break down like dino oils. (supposedly)

Supposedly, at start-up, there is always a film of syn. on the cylender walls, whereas dino all drains back into the pan. So the start-up protection is there. A couple hundred miles or so up north from where I am, if the RCMP don't use use syn, they have to use burners under the pan to keep the oil flowing or no go! Or they just NEVER shut the engines off, the guys on the next shift will "fill er up." There was a news special on this once.

11-27-03, 04:30 AM
I recently changed the oil in my 87 brougham. I used regular quaker state 10w-30. However I have read that the synthetic stuff is really good. Should I start using synthetic oil now? And if so what weight?

El Dobro
11-27-03, 09:51 AM
Hi Sal,
Just curious, why do you use a heavy oil like 20w-50?

11-27-03, 10:36 AM
The biggest misconception about oil change interval is this..... While the oil and its lubrication properties are still fine after 90 days, its the little paticles (on the micro level) that the filter cant pick up.... Suit yourself, but why take the chance!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Another thing is you cant go by color on an aluminum block engine..... With Ralphs GM, I assume it was an iron block.... And possibly the combination of excess sludge getting mixed in... The oil will turn colors faster with an iron block from the iron (affecting the color, but not the lubrication), but with an aluminum block it will only turn black when the combustion by products are built up really bad.......

Also, Ralph, I believe you have it backwards.... Synthetic molecules are the same, and it is THINNER, therefore it doesnt stick as well as dino......

Again, it comes down to..... YOU might be able to extend it that long, BUT WHY TAKE THE CHANCE!!!!!!!!

11-27-03, 10:58 AM
Hi Sal,
Just curious, why do you use a heavy oil like 20w-50?

I'm wondering the same thing, Sal.

There's no rush to change synthetic at 3,000 miles, Sal. I've been running it 3750-4000 miles, and that may even be too soon, but I'm comfortable with it.

11-27-03, 01:14 PM
Okay, I'm forgetting which oil I used. I went to Jiffy Lube and they gave me the recommended weight. What should it be in my car? I forget, actually. I'm probably wrong and it's not 20w50.. It's more likely either 5w50 or 10w50..

At any cost - if you owned my car - which would you put in? I just want whatever is best for the car. I'd like it to last me a LONG, LONG time...

11-27-03, 01:39 PM
I think a 40 weight oil would be fine...... Why are you using such a heavy oil......

10w40 should be OK... I would use 0w40 for M1 synthetic

Night Wolf
11-27-03, 05:03 PM
in my manual for 1993 Cadillac..... which also covers the non-LT1 Fleetwood, it says 10W-40 should never be used in any Cadillac at all..... my enigne says 10W-30, so that's that I use...dosn't it say on your oil fill cap?

11-27-03, 07:33 PM
Take in mind that synthetic is quite a bit thinner, especailly M1..... The M1 30 weights are very thin even for a 30 weight..... The 40 weights are a better choice because they are closer to a dino 30 weight...

El Dobro
11-28-03, 09:23 AM
The only engines that GM recommends an oil over 30 weight are the diesels, nothing else. Even the Corvettes, which specify using Mobil 1 since the engine oil coolers were dropped, use 5w-30 synthetic, nothing heavier.
Cadillac engines used 10w-30 for average use until, I believe, 2000, then it went to 5w-30.