The colder intake air may have tended to reduce the total burn of the vegatable oil. May contribute to some ring gumming.

An outside possibility but what we used to refer as carb heating to reduce icing of the carb is not available on these diesels. Although I see no reason some form of it could not be engineered in. Diesel has a much faster flame front than vegatable oil. Any real lowering of the combustion chamber temperature is going to have some effect.