I have a '97 with 150k miles on it, and its had some oil pressure problems, I wonder if the original owner had run conventional oil, but the previous owner had used synthetic (I don't think the previous owner really ever had driven the car much, I think it just sat in his driveway waiting to be fixed up).

I put in some 15W-40 Shell Rotella-T heavy duty oil, will this cause any problems? The oil pressure is high enough to be acceptable now, but the manual in the glove box says 100 times not to use anything besides the normal 5W-30 or 10W-30 (I don't remember which). Are there any problems or drawbacks to using a heavy oil like this?

 

I think that the oil pump drive gear will be really stressed by running high viscosity oils. Stress it enough and the teeth will shear off and you'llhave no oil pressure.....

 

Once it warms up it idles just below the 20 mark on the dash, which is right around where my buddies' LS1 Camaros idle at. While its cold it idles at 60-65+ (I try to let it warm up before I drive it).

Do you think I should put in a lower viscosity? I don't know what was in the car when I got it, but it would idle at 5psi or less (just going off of the gauge here, no OBD-II readers) once it was warm and that seems way too low to me.

 

Chevy small blocks are ok adhering to the old rule of thumb of 10 psi per 1000 rpm. If you had 5+psi at idle with standard weight oil you're fine.