Eh, was gonna throw in my 2 cents, but these guys seem to have it all under wraps. spark plugs are the best indicator of an issue, imo... sandpaper and a feeler gauge and cleaning them up, then monitoring them can tell you if you have an issue in one or more cylinders. Helps tell you if it is one issue or the entire engine. Also, since you have them off, Harbor Freight has an inexpensive compression tester.. I mean.. may as well at that point.
Burning fuel is an indicator... even in an old engine, and not a huge deal, since you clearly are on top of it... heh.. I bet it will be easy peasy to get those running better and cleaner.
R
I don't like using sandpaper since it is possible that grit will rub off get deposited in the plug base. That grit will eventually end up in the combustion chamber and can cause cylinder wall scratching.
The plugs for that motor are relatively inexpensive. If you think they are an issue, replace them but I don't think they are the problem, they are just a symptom.
It would help to compare all the plugs to each other, and kept in order to identify what cylinder they came from.
Knowing that wet plugs are limited to one or two cylinders as opposed to all or all on one bank can tell a lot.
For example, one bad thermostats will have more of an effect on the plugs in that bank.
Two fouled plugs next to each other can indicate a blown head gasket ( or worse).
Too much oil or two rich a mixture will affect all plugs.
A bad/clogged O2 sensor will cause a rich mixture especially at low revs. That will affect all plugs.
A single fouled plug can mean several things, one that is often overlooked is a bad ignition coil or plug wire. Usually but not always, this condition will affect performance under load or low speed rough idling. The same symptoms can occur with a bad, dirty, stuck, injector.
So,, are all plugs oily? If so, are you sure it is oil and not too much unburnt gas?