Mine was bought up on Mobil 1 and then weened onto Castrol a few years ago.

I use Castrol RS 10w60, and change it as soon as it starts to darken.

Not cheap, but then neither's a new engine.

Get mine from Brown Brothers in Dartford in SE London.
Approx £128 for a case of 4 x 4 litres last time I bought.
Good stuff, maintains remarkably good pressure when hot.
Hope i've been helpfull.