I've got an 84' S10 with the 2.8 V6, the manual says to use 5W-30 or 10W-30, the engine has well over a one hundred thousand on it and I have 5 extra quarts of Chevron 10W-40 I planned on trying in it, however, the manual says not to use 10W-40. Is this just for gas mileage or what? Is it safe to run the 10W-40? Thanks