Until today, I have always just drained the oil from the mowers, changed the oil filter, and then put some new oil in. Well, after reading the posts a few weeks ago about how some of you guys drain the oil, run new oil through it, and then drain it again it has made me curious to try it. I tried it today and I could not believe the gunk that was still coming out. I just left the plug open and dumped two quarts new quarts through it after draining the oil and the stuff that came out was unbelievable. I will definetly be doing this for now on.