I'm shocked! An oil question that has posted well over 24 hours with no replies!
Since noone else has, I'll offer my $0.02. The reason for thin oil, is so that it will flow when cold. This, of course, would apply to the oil at its coldest, which would typically be the first start up of the day. However, are sure that you NEVER go anywhere outside your normal schedule where a Canadian winter startup wouldn't happen. Do you ever go to a party in the evening for a few hours? When it's cold outside, it takes only an hour or so for the whole engine including the oil to get very cold.
If you are totally confident that you would never stray from your described schedule, you could probably get by with it. However, my thought is, why would you want to use single weight under any condition? There was a time when the multiweights had a lot of problems associated with them. However, with the excellent lubricants we have now, multi-weights selected properly will serve extremely well.
The biggest problem is when using a spread of 30. Example, 10W40. There is an additive called VI, which is the viscosity index improver. I understand that to make the stretch from 10 to 40, there is a disproprotianately increased amount of this additive. Too much of this additive can cause severe carbon problems, this is aggravated in small engines where the temperatures are higher.
If you were to use, say 15W40 for your application, you would not have to worry about an occasional variation in your routine.
Again, my $0.02.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in