Which SAE viscosity grade is most suitable for a heavy-duty diesel engine operating continuously under high-load conditions in hot summers and moderately cold winters?
SAE 15W-40
The oil must flow at moderately cold winter start-up and still stay thick enough at high summer temperature under heavy load, so a multigrade oil is needed.
In an SAE grade like 15W-40, the 15W is the cold (winter) rating and the 40 is the hot-running rating.
SAE 15W-40 is the standard heavy-duty diesel engine oil: the 40 grade gives a strong film for high-load, high-temperature running, while the 15W lets it flow at moderately cold starts.
SAE 40 is a monograde with poor cold flow, while 10W-30 and 5W-20 are too thin at high load for a heavy-duty diesel engine.
Hence, the answer is SAE 15W-40.
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