I was thinking of trying some castrol fully synthetic (I already use Shell ultra 100% synth.)
Unfortunatelly, here they only stock 5w-40 or 10w-50...
What would be best? Summer is avg 30c and winter avg 12c (in the morning maybe sometimes 0c, but not very often.)
Should I aim low or high? How big a difference is the 5 or 10 SAE grades?