: Using synthetic oil in a Northstar-Why does Cadillac recommend 10W30?

12-11-07, 11:38 AM
Just boought a 1999 DeVille. Owners manual recommends 10W30 non-synthetic oil. Says ONLY USE 5W30 if temperatures are below 50 degrees F.
Have a 1999 Corvette. Factory fill is with 5W30 Mobil 1.
Would like to use synthetic oil in the DeVille but wonder if since it has 119K miles on it this might not be a good idea. Don't want to start oil consumption on the 8 year old rings.
Why does Cadillac recommend 10W30 and not 5W30?
Will synthetic oil meeting the Corvette specs hurt or help the seasoned Northstar engine? :hmm:BIGHANK

Crown Vic Owner
12-11-07, 12:00 PM
I dont even believe mobil 1 is a real synthetic anymore. Dont take my word on it, but i remember that i promised myself that i was going to use a high quality synthetic in the caddy and i dont even use synthetic, i just use whatever decent oil is laying around and change often.

The thing with oil consuption at the rings on these cars is that they eat oil no matter what due to the agressive cylinder wall crosshatch. These motors are extremely well lubricated at the cost of consuming a little bit of oil.

I personally would run mobil 1 in my northstar without hesitation, of course it may burn a tad more oil but that could always be slowed down by a few WOT treatments and rundowns to get rid of the carbon that causes some oil burning.

I believe they recommend 10-30 because of how hot the northstar gets. If you can use 10-30 synthetic, id use that.

Just make sure to check the oil often.

12-11-07, 12:10 PM
If I remember correctly, '00 and up Northstar has flat typed valves or vice versa that's why from '00 and up is recommended 5W30 . Still, I don't think you would get too much difference whatever you use

Crown Vic Owner
12-11-07, 12:34 PM
00 is when the LD8 got less agressive cams than the l37

12-11-07, 05:00 PM
Pre 2000 Northstars have flat tappet lifters, thus the need for 10W30. In 2000 GM went to roller followers which allows for the thinner 5W oil. On a pre 2000 I'd stick with 10W. I still use it in my '03, my daughters '02 SLS and my other daughters '04 Grand Am (FWIW).

Re: oil consumption, go to the Technical Archives at the top left of this page and read up.

12-11-07, 05:28 PM
bighank............Please, before this goes any further, make a stiff drink, sit down, and go through the archives for N* oil requirements, search threads and spend hours learning.

Your manual probably allows you to run 5W-30 if the temperature is anticipated to remain below 50 for most of that oil's life. Does it specifically say "no synthetic" or does it say something like "use only GM recommended oils....."?

There is no reason on earth why you can't run your favorite brand of synthetic 10W-30, 24/7/365, (unless you live in Antarctica). N*'s DO NOT, as a blanket statement, "eat oil". (I get 3,500 mi./qt., and 3/4 of that is at 60+) I run Pennzoil Platinum, You run Mobil 1, some run Royal Purple, a few run AMSOIL, some run Quaker State, BUT the automobile owner who has the dealer or quickie lube shop do their oil and filter changes runs whatever someone else puts in their engine. Today, the number of people who do their own oil and filter work is very small. So 90% of the cars on the highway are going to blow up tomorrow? Batcrap. 99% of the people in this Forum bought their car used, and have no idea whether the previous owner used 3-in-1 or 90-140 axle oil. It's all smoke and mirrors.

Go back a day or so and read the latest GM thinking on oils and ZDP in one of AJxtcman's posts, run the oil recommended for your car, and don't sweat it.

12-11-07, 08:21 PM
Oil pressure is maintained in an oil system by what I would refer to as "controlled leakage".
Oil leaks out past bearings, and on some engines through oiling jets, seals and what have you.
The oil pump, pressure regulator, components and controlled leaks all maintain the oiling system.
When you change the viscosity you are changing the flow characteristics of the oiling system.
The reason it can be different between cars is because of how the oiling system is designed between two totally different engines.

I hope that makes some sense.
I am pretty tired right now.