Had a hard time finding Mobil 15W50 oil for my Toro ZTR Hydro pumps. Dealer does not have it. wal-mart dos not stock it. Auto parts store I use to get it from no longer stocks it. Found it a K-mart but was on the clearance rack. Is Mobil in the process of discontinuing it, if so, what do you use in place of 15W50?