Personally I think that while
Amsoil makes a fine product, but I also think it is over priced. I use Mobil 1 in the Cobra, and have no issues with leaks. I change it once a year (about 5000) and change the filter (K&N) a couple of times a year. I just top it off now and again. I just had the valve covers off the motor (it has about 13,000 on it) and you could eat off the inside of the motor. I looked like they where just tanked and blasted. 5W-30, year round, for me yeilds cold, fast idle pressure of about 80, never goes below 30 no matter how hot at idle and picks up < 10 psi per 1000.
I watch for sales on Mobil 1. Usually I can get it for about $4 a quart. I just buy a case when it is on sale.
On a side note, my local NAPA just starting carrying
Amsoil. But the manager didn't think the relationship was going to last because of the price.
I think the bottom line is all the major synthetics are a good product. I think the differences between them are small and probably insignificant. With that thought process, I buy according to price alone.
Rick