: 15W-40 Oil?

04-25-04, 07:23 PM
Hey, my dad's mechanic and I recently changed my oil on my '99 ETC and my dad's mechanic used 15W-40 oil. I told him that the Caddy uses 10W-30 oil and he said that its better to use 15W-40 in California because it gets hotter a lot more often and the higher grade oil will protect the engine better. Is he right, or should just use the 10W-30? Thanks for any thoughts.

04-25-04, 09:37 PM
Uh....he is wrong. The engine was designed for, developed and validated with 10W30 oil.... Believe it or not ,the engineers that designed/developed/validated the engine know that it might get hot in California...and Arizona, and Texas and Florida...well, you get the idea. The engine has been tested under hotter conditions than you are likely to EVER encounter (try towing trailers out of Death Valley up 14 mile long hills when the ambient temperature is 120 F....I have done that with a Northstar engine with 10W30 oil in it and it was fine) and 10W30 oil is the correct oil for those applications.

Leave the engineering of the lube system and the oil recommendations to the engineers that designed the engine and know how it operates....not to the corner mechanic. 15W40 oil is not a recommended oil for the Northstar engine.

04-26-04, 12:58 AM
The most engine wear is on startup. The thicker the oil the slower it flows at startup. You are just wearing the engine out sooner.
For any normal use (even towing if it's in the rated weight range) it's counterproductive to use heavier oil than recommended. It took a while to convince me but I'm a believer in light oils now. Plus you get slightly better gas mileage.

04-26-04, 01:48 AM
Thanks for your answers. I'm gonna use 10W-30 next oil change, but do you think I'm gonna hurt anything by using the 15W-40 this one time?

El Dobro
04-26-04, 04:33 PM
I would change the oil. You can leave the filter on.

04-26-04, 09:11 PM
I agree that 15W-40 is not a motor oil that should be used with the northstar engine. As I understand it, the 15W-40 is designed for diesel engines and like motorcycle oils, have no friction modifiers in it.

I would change the oil ASAP, if it were me.

04-26-04, 11:21 PM
Do you know the rating of the oil? If it's rated OK I'd leave it in, 2-3k miles won't wreck the engine. I believe it should be SJ.

04-26-04, 11:43 PM
The only thing I know about the oil is that its Pennzoil 15W-40 and that we use the same kind of oil in the diesel pickups that our company owns.

04-27-04, 12:41 AM
Look at the label and see what it's rating is...SJ is OK to leave in.

04-27-04, 01:57 PM
SJ is an ancient rating. SL is the current rating that you are looking for.

The oil won't hurt anything for this fill. Just switch to the correct weight when you change next time.

Chances are if it is a heavy duty diesel oil it is rated SL and is fine in the Northstar.

What you REALLY want to look for on the can is the GF3 or GF4 rating signifying for use in gasoline engines. This , in effect, supercedes the SL rating as the GF3/4 rating includes SL and the industry required fuel economy friciton modifyers for gasoline engines.