Can I use 10w40 in my older car in the summer and 10W30 in the winter?

(2011-03-27 14:25:11) 1. tommaso Says: Hello, I have a 1998 Trans AM LS1. I live in NYC, so the summer is really hot like lower 90's and the winter temperature can drop down to the 20's.

My engine is a little older and this can be evidenced by having a low oil pressure at a warm idle (like 18-20 PSI once fully warmed up, and will increase with rpm's).

Is it ok if I use 10w30 in the winter and 10w40, or do I have the numbers mixed around?

thank you.2. MUSTANG_MAN Says: Sounds like a good plan, or even use Castrol GTX 20-50 in the summer..3. cjo84070 Says: Just use 10w30 all year. Use a higher mileage engine oil. Changing to a 10w40 will cause lower fuel economy. Tag: Can I use 10w40 in my older car in the summer and 10W30 in the winter?