If you find your local ACDelco dealer they may give you a good price on it. I've been buying it from Rock Auto for a few years now, last was $8.15 per quart delivered. Then when I thought my order was lost, I bought a couple cases from Summit Racing for $8.99 a quart delivered.
Plan B in case ACDelco got too hard to get or expensive was Shell Rotella Gas Truck but that appears to be gone now. So next down the list on viscosity was Quaker State. We're going to try it out on the next oil change and see if the motor rattles on start up like M1 used to do.