I for one use Amsoil in all my cars, front to back, including a really keen-o air filter on my Subaru Outback.. I find them to be high quality products.
I have heard that the best route is to do a break in for about 500 miles at various speeds, not racing it, then change the oil from petro and a new filter to synth oil from then on. Since the motor is"tight" when new, there should be some break in wear.
Mobil 1 is quite good, but Amsoil is probably better. Longer periods between changes, like 10,000 miles for Amsoil. I think 5000 miles or so is fine with a quality synth oil. It's still cheap insurance if you plan on keeping the car for 150,000 miles and beyond. With a turbo, oil is all the more important.
With the way Amsoil is pushed I don't think I'd ever buy it either way. I usually put Mobile 1 in the turbos and Castrol GTX in the naturally aspirated cars, for the most part though oil is oil, the important thing is to change it on time and use a quality filter. Amsoil just screams scam to me.
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