Someone.. Mobil 1.. tried to get fancy with airplane oil once and it only cost them about 30 million in engine overhauls! There be no sythetic oil running in lycomings and continentals that is certified. They've tried and all run away with their tails between their legs!
For one thing, synthetics are only partly compatible with petroleum oils and, more importantly, with seals and gaskets that are used in aircraft engines. There is no doubt that when switching aircraft engines over to synthetic that the likelihood of seal and gasket failure is quite high. The longer the engine has been operated since overhaul, the higher the risk. Anyone who says otherwise is not being straight with you.
Another problem is that synthetics do a VERY lousy job of holding contaminants in suspension, which is a big issue with an aircraft engine that is not running a filter (most don't). They run on a screen only. If you doubt that, ask the synthetic oil people why they don't sell an approved AD aircraft piston engine oil.
That said... 9.9's (well at least Paul's and mine) don't have an oil filter either. I will not use a synthetic oil in it, period!