We use hours to monitor how often we do service work on our farm equipment too. Some pieces of equipment are idle more often while others are run under heavy load most of the time so it's not really the same. If a piece of equipment came with an "on-dash" gauge that measured fuel use, that'd be the most accurate. Then it'd be like, once you use 500 gallons of fuel, time for an oil change instead of 500 hours. Just an example. Then a lot of service manuals recommend shorter change intervals if you're using diesel fuel that has higher amounts of sulphur in it ( I don't know if you can still get anything other than ULSD though ). I don't know if using higher octane gas would have a similar effect, wearing on a gas engine less.