SAE 30...but not 15w-40???

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Mar 8, 2003
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.
The oil channels are not big enough to flow a 40 oil at operating temperatures.
I put 100,000 miles on a '94 version of that truck. From new, oil consumption was on the high side of normal (1qt/2000 mi) and never varied on 10W-30. This engine does not have unusually tight clearances. CAFE is one reason for the oil recommendations for the light-duty versions. Another factor could be that, especially in '92, that dino 10W-40 was not holding up well in service and was still a very popular grade and they wanted to make clear that 10W-40 was not to be used. With that many miles on it, use the 15W-40.
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