I have been using Amsoil 15-40w full synthetic diesel motor oil in my rig for two years and have had no problems so far. There ain't no API circle on the bottle and that bothers me. I was told that it cost about $400. k to get that certification so they did not want to suffer the expense and just rely on word of mouth and extensive advertising instead. Their choice, but $400. k is chump change for a company this size, so I smell something wrong not to mention warrenty implications from auto mfgs. Went into a NAPCO store and saw the Royal Purple oil and the guys there all raved over it. So I called the RP 1-800 # and talked to one of their engineers and asked him about his product. Well of course it is better than anything on the market and there API certified right on the bottle. Well hear it comes the guy said through independent tests that they beat Amsoil in every catagory including the 4 ball stress test by a long shot. By 1/3 less wear spot and the spot was polished after the test. I as I recall I have never seen the Amsoil grafts show RP in their comparisons and by the way they are competitive in price. ( mail order from oil town). API certification gets my vote and makes my warrenty feel better, hear say is just like weight loss pill adds. b:--) I am a retired Exxon guy and RP is in my opinion a superior product because they passed the tests. :-{} Oo. 
