Well, I just got myself a new truck: '92 Chevy C1500 W/T 4.3L V-6 with HD 5spd. 87,000 miles Anyways, looking at the oil spec chart in the owners manual, it lists the following (book covers all 1992 Chevy pickups). Use 10w-30 when in "severe" operating conditions for light-duty vehicles, or for all conditions (except below freezing) in HD vehicles. Use 5w-30 all-year round in light-duty vehicles, but only in cold-weather in HD vehicles. Lastly, it says at temperatures above 40 degrees, that the use of SAE 30 is acceptable. Yet, on the very next sentence, it says not to use 10w-40 or any other grade not listed. My question is this: How would a 15w-40 not be as good as SAE 30 in temperatures above 40 degrees? Does anyone know Chevy's logic in this recommendation? Thanks.