Have you checked the seal on the oil cap? It tends to get hard and allow some oil to escape. Happened on my 850, but there was visible oil around there to give it away.

The cataylst may be taking care of the oil, or the evap system may be removing and burning the vapors.

How is the coolant level? Is it normal? Any oil in the coolant? If so, there could be a breach in the head gasket.

Don't know how many miles you have, but it is possible as the engine ages, that it will start to burn oil, if it did not previously. It's not a horrible thing, provided you check it often enough. Can impact the cat. over time, just like an overly rich mixture with unburnt fuel.

Are you running 10/30? Maybe try 10/40 depending on where you are. I think the manual has a range for 10/40, I could be wrong - please check.

Maybe a piston ring has seized and is the culprit - no difference in the plugs between the 5? All the same condition?

Just some ideas to ponder.