Originally Posted by y_p_w
Originally Posted by gfh77665
I use straight SAE 40 wt. for years in SE TX summers. Worked perfect. Zero issues.
I thought that the big issue with API rated straight weight oils is that they're not required to meet many of the requirements for emissions protection for modern cars spec'ed for multiweight oils.
Also - the definition is somewhat different compared to what it would have been 40 years ago. It might even be a Group II base oil that might technically meet something like a 20W-40 requirement.
But I do agree with the other comment that the most important consideration is probably the cooling system.
You are right, but it was high quality Pennzoil. Of course CAFE would not approve such, but it worked perfect in the truck. I may have lost a 0.5 MPG or so, but if I did I did not even notice. All sounds, MPG, oil pressure, etc, stayed almost unchanged. I did it for years and the truck still performes perfectly.