I have an older Troybilt ('94) lawn tractor with an Eaton 851 hydrostat. The oil cap says "20W oil" on it. Do they mean regular 20w engine oil? If so where can you buy it? Is there something else I can use that is 100% safe for my hydrostat? I have had it since new and have had zero problems with the unit and don't want to put something in that is going to damage it in the long run.