I have used
Amsoil in my 390 FE, my daily driver, my truck, my motorcycle (Built Buell), numerous racing motorcycles (all air-cooled) and in everything I drove when I lived in Anchorage. I always changed it every year, or more frequently if I drove the vehicle a lot. I swear by it.
I have used other synthetics and the closest to
Amsoil was Torco. Redline isn't bad either. I also used zero wt. Royal Purple in one of my race bikes, worked incredibly well and increased HP. Couldn't seat the rings with it but it worked great after seating with dino
oil. Dumped it after each race and didn't blink an eye at the cost.
I have usually changed the
Amsoil in my daily driver after 12,000 miles but change the filters (Amsoil also) every 6k. No problems...... and it starts well in our cold winters and hot summers.
Don't know where you get the $10/qt price. I sign up for the Preferred Customer deal ($20/year) and get it for dealer price. It costs $7+/qt retail but I pay around $5.75/qt. And, yes it's available in stores or you can go to their website and find a distributor near you. Not hard.
All good things take a little effort to procure. Amsoil has been worth it for me.
Paul