Hi guys.
I would just like to get your opinions on the following.
In the service manual of our datsuns they state that engine oil of weight 30W or 40W must be used and with the API specification of SF.
On my previous thread I posted on engine oil some guys replied by saying they use synthetic oils.
I am using Castrol GTX2 with the specs of 20W-50 and SL as the API.
Is it possible that we are using too advanced oil for the engine in the way that it will cause more harm than protection? I read once in a cars service manual "do not use synthetic oil".(cannot remember what car)
This is just out of curiosity what I want to know what you guys think.

My personal opinion is that I think we will get longer engine life with todays oils as when we use oils with the specs they give in the manuals.
Meaning the engines was build to use the lower spec oils of those long gone years but we are using far more advanced oils increasing the overal engine protection.