Hello. My observation between these oils is not scientific, just my own use
I have used both of these products in my 07 Acura MDX with a 3.7L. I must say I like the 5/40 better. Runs smoother, than the Castrol, in 5500 miles the Motul didn't burn a drop, the Castrol on the other hand 1/2qt in the same time frame. So when someone ask, which is better, I have to say Motul. It's also loaded with more mfg approvals.
I have used both of these products in my 07 Acura MDX with a 3.7L. I must say I like the 5/40 better. Runs smoother, than the Castrol, in 5500 miles the Motul didn't burn a drop, the Castrol on the other hand 1/2qt in the same time frame. So when someone ask, which is better, I have to say Motul. It's also loaded with more mfg approvals.