Thanks PHIL,

I've got your point. I use truck for city driving and occasionally up to the mountains' tops in the winter.

There is no towing in the plans so far...

Also here

http://www.worldsbestoil.ca/which-30-weight-oil.php

says:"Although a vehicle that is recommended to use a 30 weight oil can also use a 40 weight oil, it is usually not needed. You will gain absolutely no benefit from using a thicker viscosity oil if it is not needed. The only time we recommend a 40 weight oil, such as AMSOIL's 10W-40, to a customer in a passenger car or light truck application is if the vehicle's engine is excessively worn and consumes oil at a higher than normal rate or if the vehicle is being used for very severe duty, high load, high temperature applications."