Well on a T 15 I would run a medium to a hot plug. Any brand I could get that was the proper length like McCoy, RB and so on. Even thaw I like O’Donnell Race blend fuel I do not like there plugs as some glow plug starters do not fit properly.

OS plugs will work but are not 100% correct, as they are shorter then most others. If you ever take your head of a none OS engine and put a OS plug in and then the proper one you will see the difference.