I'm using Valvoline VR1 Racing 15w50 in Stage 1 but it's very expensive (about 21 Euros / Litre). I would like to try Mobil Motorsport but it's not available in Portugal anymore (since Mobil was joined to BP).

Castrol RS 10w60 is a good one, already used it, but again it's getting hard to get here (they have put it just as 10w40).