I have spent 30 years running
I have spent 30 years running a fleet of Diesel engineTractor Trailers in Cold Weather, along with a handful of pickup trucks with gas engines. We oil sampled our engines with a lab every 20,000kms, and determined that if you use a high quality synthetic oil (Amsoil) with a high qualtiy filtration system (spinners on the diesels), along with quality oil filters, and high quality oil filters on the gas engines, we got millions of kms from these engines, and changed the oil when the Lab told us to - the synthetic oil, when kept clean, ran for 80,000 to 120,000 kms between oil changes - we had 1 Mack engine run 400,000 kms with no oil change, just filters and lab analysis every 20,000km - and the oil was still in good order at 400,000 km, we changed it just on principle. Cleaning the 100% Synthetic Oil was the best investment we made, high grade filters and Spinners saved us many many $$'s over the life time of these vehicles.