Cold air is denser.. a given volume of cold air weighs more than wrmer air. Fuel-Air mixtures are calculated by weight.
So if the volume of air weighs more the mix is leaner since more molecules of air are mixed with a given amount of fuel. Generally, lean mixes burn hotter (more power :). Rich mixes burn cooler and waste fuel.
But that has nothing to do with how much oil/lubrication is in the mix. Oil is a third component of the mix. Air-fuel-oil.
Adding more oil will "lean" the mixture further. Oil takes the place of fuel. Lots of oil mens less fuel. You don't risk lack of lubrication. The problem here is lack of power and other lean-mix problems like pinging.
If i experienced engine problems, like noises or hard starting or lack of power or if I was just interested in how the bike ran in cold weather, i would do plug chops to determine if i was running rich or lean.. If i was running too lean the cure would be a larger main jet.
Since more oil makes an engine run leaner, first i might try adding less oil. This would have the same effect as installing a larger jet ( less oil will richen the air:fuel mix). If this experiment made the bike run better i might then look seriously into finding a larger main jet.
But the bottom line for me is if it aint broke, don't fix it.