How often do you guys recommend changing the Oil22 in your Trane Centrifugals? My annual testing from Trane comes back perfect every season and the oil looks like the day it was put in. I know Trane recommends seasonal testing and change the oil when tests dictate but the last change on this 1470 was about 6 years ago. The filter gets changed every season. This machine runs about 2500 hrs a year and has had no issues since start up in 2007.