I'm on AeroShell 15W50, too. The right oil and its additives is a rather religious topic. What I found interesting is this article from one of the US oil analysis labs:
About Aircraft Oils
It basically confirms what Peter said -- doesn't really matter which oil as long as you do not run it past 50h.
The previous owner of my plane (TR182 with O-540 + turbocharger) was on a 25h interval, he said it couldn't harm and that after about 25h, the engine started to consume oil which he thought means that some characteristics must have changed. My airplane oil after 25h looks worse than my car oil after 3 years...
On a side topic: what are the top mail order aviation supply outfits in the UK? I noticed that AeroShell is a lot cheaper in the UK (found transair.co.uk) and shipping doesn't cost much.