amsoil 15w40 diesel and marine oil all year I run it 15k miles and change it. raced with it past 2 seasons 0 issues.

I use ROTELLA, DEL-VAC, or DELO 15-40, Which ever one out of the three is on sale:tu:

Motor Oil Question
 

Rotella all day everyday. The old goat just turned of 340,000miles. Have a few oil leaks I been wanting to fix so I pulled the motor decided to go ahead and replace bearings and piston rings. You can still see the hone marks on everything. That is because it is a well oiled machine.


Brandon

ams 5-40 during the winter and ams 15-40 during the summer whether or not starting would be any better or worse with the 5-40 i don't know, but in theory it should flow alot better
at colder temps

It does flow better, bobcat! At least during the important cold start phase.
It's easy to compare- try to suck honey through a straw when cold. Try again at room temperature.
Same principle.

The most important part is not increased oil flow but lowered resistance to start the engine. At 32°F starting current is increased by 100%. At 0°F current is increased up to 250% with a standard oil.*
You can lower that resistance a lot with a thinner winter oil to enable a safe start. So a thinner oil is giving you a start insurance during cold weather.


* You find these numbers in the bosch automotive handbook. Bosch is the world biggest manufacturer of starters.