I am changing my Hydraulic Oil and the manual explains to drop the oil filter and allow to drain. The manual says to replace filter and fill with approx 4 quarts of 15w50. The issue I'm having is it only appears that a quart to quart and a half has drained. I did check the level prior to emptying to make sure that it was full as I always do. Do I need to drain the 2 pumps separately. Gravely 148Z Model #991021 Serial #000203. Please advise