Mike; Most years it is three oil changes, this past was four changes due to the way the grass grew. I think is about $15.00 a change, I use the Kubota filter and the Kubota oil, it is just easy for me to pick up the oil while at the dealers. testing would be a good idea, but the tests I have seen cost at least $15.00, so I just change the oil for the same price.
Will