It's something that bothers me, I have a mini moke that has done 8 miles (on fresh oil) in 2 1/2 years - just wondering if I should change the oil, obviously the engine is an 80 year old design so god knows what oil they had in those days. I have new oil, I think it's 20/50 mineral (but it's definitely the recommended one) but I think it's wasteful and bad for the environment to just throw away the old stuff on the basis of age. Most of the "advice" on the internet seems to be from oil suppliers. I know that on commercial vehicles they test the oil rather than relying on age or mileage.