I use Castrol in my '68 TR6. When I bought my 2011 T100, 20W50 was recommended. Then Triumph switched from Mobile to Castrol and Castrol does not make a 20W50 motorcycle oil. So now Triumph recommends 20W40 for the Bonneville family. Because I ride in the heavy traffic in the Southeast with temps approaching 100F, I use fully synthetic 20W50, which is not Castrol.
I don't why Triumph dropped Mobile1; but I do know that Mobile changed its additive package to one with less anti wear formula. Castrol has a better anti wear package as does Amsoil and several leading motorcycle oils.
I do run 20W40 in my 800XC; but it is liquid cooled.