Ive been trying to find 80w90 gl 4 for my saginaw 4 speed but i cannot find it anywhere, but i have been able to find 85w90 gl 4. Is there much of a difference between these two. I live in california so it is never really that cold, and my chevelle has no heater so i dont ever drive it when it is cold. Any thoughts on what i can use? Ive used the search and even google search and could not come up with a definitive answer.