My engine dislikes xxW50 grade oil - I tried Motul 15W50 oil earlier this summer thinking that is would handle the sub-tropical heat here better than my Shell Helix Ultra 5W40 grade oil. How wrong I was. Acceleration was noticibly more sluggish - so I dumped it after only 200km and filled up with the Shell again. The difference was immediate - engine back to its old self. And I've had the same experience with a couple of my motorcycles, both on the road and on the dyno. I should have learnt
No more high viscisity oils for me.