Last week I borrowed an elderly cousin's small 4 cyl. Ford diesel tractor for a few days. Before returning it to him I asked how did he know when to change the oil since it didn't have an odometer. He replied that you could use the hour meter, but he generally just waited till the oil felt gritty between his fingers. He said he had not changed it in about five years!! Needless to say, I changed the oil and filter before returning the little beast.