Why not R?? R needs to be warm enough to work properly, below that it’s like treacle and ain’t working. If it gets too hot, it breaks down and is ruined. Between those parameters it’s brilliant stuff. But after the race, it will gum up engines if left in for too long, and it absorbs moisture. Bottom line: R is great but is very high maintenance. Good modern synthetics are also great, without the diva high maintenance.