You would think that that would be the case, but sometimes the oil drain back holes get gummed up so it doesn't matter what viscosity you use because no oil can drain back anyway. Also, there aren't enough drain back holes so as the piston moves down in it's stroke, and the piston rings scrape the oil off of the cylinder walls, there just isn't enough area for the volume of oil to escape so some is left on the piston walls which gets burned up during combustion. I also heard somewhere that our piston rings aren't the best things out there so there is some slop in them that allows some oil to get by. We're just hosed.