Which oil is best to use for my 5jj r1? fully or semi synth?

In my R1 5VY I use Maxima Extra4 synthetic 15w50, and I get it from Andy Bates recommended by him too!

Been using halfords fully synthetic motorcycle oil in my 5JJ R1 for the last 15,000 miles, its done 17,000, changed every 3,000 max, and the engine still sounds like new, and the oil comes out almost as clean as it went in. The bike gets no mercy, and is thrashed every ride out.

For me it's always been silkolene comp4 in my car and in my road bikes.