NAPA has some SAE 50 but only two brands. Purple Power and some for racing. I'm not sure if there is any difference or not. I went with GL-5/MT-1 to start with and I may try SAE 50 later. There are many threads hotly debating which lube is best. Some make sound, information-based arguements and some go on personal experience or 3rd party info.
It seems like all 3 (GL-1, GL-5/MT-1, and SAE 50) will do the job. If you aren't sure, drain your old oil and try to get out the old gunk. Try a lube for a while and then drain it and check for new gunk. If I win the lottery I promise to set up a system testing all 3 in a very scientific test. Until then, Let the battle continue. Be sure whatever you use is yellow metal safe, MT-1.