The Yanmar dealer here in St. Pete recommends non detergent straight 30wt. There is not a clutch per say but what is called a "cone" nine times out of ten if your tranny is slipping that is the problem. Have you checked your shift linkage? It is very critical that the linkage maintains pressure on the shift lever of the trans. There is a barrel type piece (can't remember what it's called) that is attached to the end of the shift cable then to the shift lever itself on the trans. This has a double acting spring to keep tension on the shift lever both in forward and reverse. Hope this all makes sense?