I used the 15w40, it was £10 in Asda on special offer. Engine was smooth and quiet. I would say that after 30 minutes of hard driving, the oil pressure was slighlty lower lower (2-3 psi, so not much) than usual operating temps, which showed it was thinning out a bit under heat. This is why I went with some recommendations and got the Mobil 1. Wished i never bothered. Going to try some Pro s 10w50 and if that doesnt improve things, I'll be switch back to the GTX.