just a thought if your running twin engines try one on synthetic and the other on the usual oil if you encounter any probs switch to what ever is doing best, personally the only problem i can see is the cleaning adddatives in synthetic removing any carbon build up and thus lowering compresion on the engine some more info on the following links
http://www.buzzle.com/editorials/9-8-2006-108031.asp
http://www.howstuffworks.com/search.php