For years air cooled lawn mower manufacturers haver recommended straight 30wt oil... specifically warning against using 10-30 or whatever.... This is a severe situation and thus I took that to mean that the additives given oils to get those readings decreased the lubrication abilities...

Automobile manufacturers have been under pressure to get better ' fleet overall ' gas mileage.. " readings... there may have even been money penalties for not reaching certain ' goals' set by the government....

Usually a lighter oil will be less resistance and thus give a small increase in mileage overall...

However, that may leave you with your engine lasting a bit less time ... but no one is really collecting those comparison numbers... so the government is not upset to have the ' tradeoff ' hit the car owner in that fashion. Short sighted... but some officials are that way... now that " image is everything"...

It does make a difference in the winter some places with regards to getting your oil to bearings quickly.... and I have always been in favor of pre-oilers like the big truck rigs use... they are in it for the ' long haul' if you will pardon the pun...

I am down here in the Texas heat... I buy Delo400 in Straght 40 wt ( by the 5 gallon bucket ) and use it in my 240 and my 5600 ford diesel tractor. I could not do that in the winter if I were in a colder area...