I have a 1989 F250 with the 460 (7.5L). The previous owner always used Castrol 20-50, and I have followed that recommendation for the year I have owned this truck. I just discovered that the owners manual recommends 10w30 for 0 deg F or above or 15w-40 for 10 deg F or above. Is the 20-50 too thick? Any thoughts? The engine occasionally squeaks a little when warming up from a cold start, indicating that it might not be getting lubricated properly due to the oil being too thick.

I live in a very mild climate by the beach in Central California. Winter temperatures average low 40 high 60. Summer low 50 high 75. But the truck is occasionally used for long road trips all over the west towing a small trailer. Still, it will rarely see anything below 20 degrees or above 100.

For what it's worth, I also have a '73 Ford Wagon with an older version of the same engine (460). For that, I have always used 10w-30 in the winter and 10W-40 in the summer, per the manuals recommendation and have never had problems.