This extract is taken from the 'Operators Handbook' for the Pratt & Whitney C4 engine (B-24)
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"As a result of service experience, it has been found that it is desirable to use an oil of 95-105 viscosity, even in warm weather. Generally speaking, the use of the lighter oil results in better lubrication for the parts subject to wear and tends to reduce the amount of carbon deposits"
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Hope this helps.
Regards
Peter.