Do a few track days in my 06 STI and just wondered what the best and most suited oil i should be using, my regular oil i use is 10 40 semithetic.

 

I'd up it to a 10W-50. Millers CFS and Silkolene Pro S do these weights and are top-notch fully synth jobbies.

The 50 "hot" weight having more viscosity at 100deg+ than a 40, so will maintain the pressure through the bearings with hot oil, when hooning on track.

A 60 hot weight is considered TOO thick in modern motors.