The F115 is notorious for making oil. I know this will sound dumb, but change your thermostat. I bet it is stuck open. basically the cause is the thermostat sticks open (or poppet valve) and the engine is over cooled. when it is run cold like this, the rings do not seal properly and the cylindrs become "glazed". basically this allows raw fuel to go past the rings and into the oil.
Once you change the thermostat and get it back to running at normal operating temp again, run it pretty hard for a little bit. AKA WOT under load This will help to generate heat and the rings will seal again and all will be well.
Had this happen twice with my 115 and once with a 90. all 3 times were stuck open thermostat.
Another thing that can cause this is excessive idling or putting around. basically has the same effect.