OIl for s13

[240SOHC89]

hey all jus have a simple question, i have a 89 s13 SOHC, i was wondering if i am using the right oil, since i did my first oil change back in late 2003 ive been using 10w40, basically cuz my friend was using it on his car, ever since i stuck with it and havent had problems, bought the car with 127***, and it now has 140***, my question is what is the difference in 10w30 to 10w40 and all those other types, am i using the correct weight for the millage of my car??? i live in sac CA so it doesnt get to cold, well helps if you can Later

[grip_racer]

5w-30 sythetic or nothin!