[oilman]

You are spot on, the cold start viscosity of the oil is very important to ambient outside temps to ensure good cold flow on start up.

Once the engine is up to temp modern engines regulate their own temperature and are very efficient at it so if the vehicle is recommended SAE30 when hot (100degc) then it should be fine in most climates.

The exception is if the engine is worked hard enough it is struggling to maintain a consitant temp, this can be caused by things like prolonged heavy towing in hot climates. The answer is to go up a grade so instead of SAE30 you move to SAE40 and this will be sufficient to cope with the extra temp. The engine will try to keep the oil at a certain bulk temp (somewhere between 90 and 100degc) as this is the viscosity it is designed to run on. If you exceed that temp consistently by say, 15degc so bulk temp is now 115degc then by using an SAE40 it will be now functioning as an SAE30 is it continues to get thinner past 100degc. And SAE30 is what the engine wants.

Hope this helps?

Cheers,

Guy