OK then let me educate you on the S ratings as far as I know. Maybe others can offer their knowledge as well. First the S. This stands for "spark" ignition type engines and as you know their are the "compression" types engines that are the diesel family. There is only one diesel engine M/C I have ever heard of.
The G is the American Petroleum Institutes (API) rating that manufactures could place on their containers along with the S...therefore SG...after they tested their oils according to a strict set of tests. Don't take these rating lightly. Now according to my information, they have improved their testing methods and have therefore upgraded from the "G" through the alphabet to what you now see as SL. The SL oils are suppose to superseed all other ratings and therefore can be used in place of an SG oil. This is the reason you are having trouble finding SG rated oils. If you can believe it, my brother found a bunch of the stuff in tin cans that someone was throwing away. It was a Sears brand. I used it all in my Suzuki GS450 GA and it works just great. Here in Canada, Canadian tire sell a SG rated oil and my feeling is that it is manufactured by Esso. I will used it in the 400.
Remember, the 400 engine is a roller bearing engine and can tolerant a lot of abuse but keep clean oil in it and you can expect a long life. That's my intention.