I will use W80 in colder periods and W100 in warmer.
I don’t see any basis for that, if the engine has thermostatically controlled oil temperature.
Several tables from the oils and engine manufacturers recommend different oils according to OAT, but you are right, in flight they reach the same temps.
That’s a mystery 
For reference, Total XPD 80/100 oils “already contain an anti-wear additive similar to TEXTRON Lycoming LW 16702.”
So definitely comparable to W80/100+
Jujupilote wrote:
Several tables from the oils and engine manufacturers recommend different oils according to OAT,
Lycoming has published a table which Peter posted on a first post of this thread. It’s quite detailed.
Continental however recommends SAE30 below 40F and SAE50 above 40F. That’s all.
SAE 30 used to be W60 or W65? (not available any more in EU I think)
SAE 50 is W100.
I wander why they don’t recommend W80 (SAE40) which is somewhere between and looking at it’s parameters it should be perfectly fine if not too hot.
