I use Amsoil synthetic in everything we have. My wife drives a 90mile round trip every day to work and back. Using the synthetic oil and Amsoil oil filter extends oil intervals to 25K miles (yes, it's 25,000 miles). When our vehicles ('01 Chevy HD1500/ '05 Tahoe) were new I did oil changes at 1K miles to get rid of the break-in junk. I refilled w/Castrol 10/40 and went another 5K miles. My wife is on her 3rd Tahoe. The other 2 had over 100Ks, ran fine, and had no oil related issues (no issues related to anything).
From then on they've been on Amsoil. The oil does get dark after a while, and since the oil filters are pretty small (LS-1 engines), I change oil filters at about every 8K miles. I drain the old oil (after 25Ks), install a "cheap" oil filter and fill it w/Castrol oil. Let it run for 1/2 hour and then drain the "flush" oil. Refill it w/synthetic, install the good filter and it's good to go for another 8Ks.
I have a customer that uses Amsoil, and he never keeps track of his mileage. He has gone as much as 35K+ on oil and filter on several occasions. His truck (Z-71) came in for a leaking intake gasket. It had 150K+ miles on it w/not a bit of build up under the intake. that to me was proof that the oil does what they claim.
I use it in my jet ski, the bike, and the Corvette. Oh, my john Deere lawn tractor gets it too. The Vette doesn't see as many miles as the others, so it's changed once a year.