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A biased reply, based on 25 years of using and selling AMSOIL
AMSOIL for the top line, Mobil 1 my next choice.
The others have very good products, but (except for Redline, far as I know) they all use Group III base stocks, which can approach but not reach the capabilities of the Group IV (PAO) stocks. There is also a significant difference in additive packages.
Many years ago, in the dark ages, I ran some oil analysis on my 1985 245 turbo wagon (before I put a bypass filter on it). The oil samples showed the oil suitable for continued use after 11,500 miles (1 year and 3 times the recommended drain). I dumped it then, mostly because it was starting to look bad. Now that is the backup car, and I have a bypass filter on it. I usually change it every other year.
Without oil analysis, I'd use AMSOIL for twice the recommended drain (or 1 year) in a turbo, for 25-35,000 miles (1 year) in non-turbo.
After the initial ATF change (complete), I change what it is in the pan evry 50,000. Differential fluid stays in for life (check level from time to time) unless the car gets parked under water.
I would not try to extend the factory recommended drain interval with any synthetics other than AMSOIL unless I used oil analysis. However, any of the name brand synthetics should be easily capable of going the reommended drain.
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