IMHO: I'm wondering,
Who is Dave? (that says to use 40:1) does he know more than the people who built the saw. Ask Dave if the Owners manual has any more info that is not correct and maybe he should get out a Addendum (or ECO, Engineering Change Order) for the owners manual?
I'm also wondering why you are mixing 5 gallons of fuel for one saw? (are you cutting timber for a sawmill or building a log cabin and don't want to run out of fuel) I buy non-ethanol fuel in large quantities for small engines and chainsaws, but I do not mix 5 gallons at a time for chainsaws