evowilly,
Castrol RS is a good oil , but not the best and not the correct grade 4 an Evo. Use 10w30 synthetic such as Redline or Amsoil.
As Oli said above oils , with a wide viscosity range have a big proportion of Viscosity Improvers in them , these r basically long chain polymers which shear down (cut in2 smaller pieces) as the oil is used , ultimately the oil will degrade down 2 the lowest number in the grade if not changed soon enuf or the oil is subjected 2 alot of severe service.
Redline and Amsoil 10w30 don't have any VI improvers in them 2 degrade.
Most race cars use single grade synthetic oil 2 eliminate degrading , they also don't need multi-grades because they don't have 2 worry about cold and stop start use.
U need 2 remember that Castrol and Mobil spend a huge amount on marketing , this is subtle brain washing I'm afraid and we all fall victim 2 it 2 some extent.