When I bought my wife's Audi A4 1.8 turbo, the word on the internet was that synthetic oil was a must due to the high turbo temperatures. I've run Castrol Syntex in it since 5000 miles or so (let it break in first). Although VW/Audi would do free oil changes for the first 24k miles or so, they wouldn't spring for synthetic, so they've yet to touch the car, which has 170k fairly trouble free miles on it now. Several years later it turns out that there were major sludge problems with these engines, and now they are insisting that ONLY synthetic oils be used in those engines.
Having said that, William Wynne says synthetics make Corvair engines more likely to leak. In the Corvair I run Castrol 20W-50 during summer, and 5W-40 in the winter. Don't forget that at altitude, the oil temp is going to be in the 200 degree range anyway, so I would think that altitude has no real effect on the viscosity range one would run. Mark Langford, Harvest, AL see homebuilt airplane at http://www.N56ML.com email to N56ML "at" hiwaay.net
