I just switched from 15w-40 to semi synthetic Valvoline 2000 15w-40 . And ever since then I get a smell in the engine compartment after I stop and shut it off (kinda smells like gear oil a little). My question is does semi synthetic have a little odor to it or does it sound like something else?? No leaks under the hood at all and I never changed rear fluid since new with about 35K on truck now(havent checked level either) THANKS IN ADVANCE