A decal on my furnace blower seemed to say the motor was permanently
lubricated. The other day I got my head a little lower and saw a second
decal saying it should be lubricated every couple of years with 20W
nondetergent oil.

I think 3-in-1 is like that, but I can't find my can. I did find a
4-ounce container of Marvel Lubricating Oil. Among the uses listed on
the label is "small electric motors."

Small is relative. Would Marvel Lubricating Oil be good for a furnace
blower?