Years ago, I was an aeroshell 100 user. The Extra 300L sales and maintenance shop said: "Aeroshell 100 gives the best results". However, after pulling one of our engines apart after a prop strike at 93 hours total time, I was horrified. Plenty of corrosion, wear, varnish and some sludge. In fact, the camshaft was ruined by obvious corrosion, as were the connecting rods. 1 year, 93 hours! I switched to A/S 15W-50 Semi Syn in all our aircraft. That same engine came apart after another 100 hours for the Lycoming crankshaft AD. It was in perfect condition! I'm getting the same hot oil pressure, better cold startup oil pressure and cleaner looking oil. All good things. Here is an interesting blurb on aircraft engine oils: Avweb "oils" Both Shell and Exxon say good things about the multi grades they sell. Shell says the multi grades have the most complete additive package of the oils they sell. Exxon says the multi grade outperforms the mono grades. Phillips calls the multi grades the premium oil of choice. The article goes on to discuss mono and multi grade corrosion, additives, synthetics, super oil, and more. However, none of the company engineers interviewed give away any trade secrets. Other than Phillips seems to have fewer additives, if I read it right. After reading a BITOG post by Ed Kollin from Camguard and the sidebar on the above article, he seems to favor Phillips so he can add his product. I still favor A/S 15W-50 due to real world results. My Cardinal engine failed prematurely on a diet of Phillips XC, with massive internal corrosion. Sure, I have limited data points. However, the premium oils seem to be the right direction for my flight department.