Air filters

I'm continuing the tuning process for my 77 KZ750B and have some concerns about the existing air filter. Oiled foam elements are a new experience for me. I'm unable to determine what good looks like. How wet and what grade of oil should be used. A heavy slathering of gear oil or a light mist of WD40? The whole concept seems so variable- too much so.
I'm considering replacing it with a paper substitute.
Any advice or experiences?

Please check the ownwer's manula or the factory service manual for you specific bike.

Generally, oiled filters use the same oil as the engine, or a manufacturer specific oil. Either way, a very thin coat is all that is needed.
The K&N and APE individual filters I have used recommend a really sticky filter oil. Flexible foam filters I have used require applying a little clean engine oil and the gently squeezing out the excess.

Neither has to be done very often, and are not really much of a problem.

I have used the same K&N air filter for around 35 years with a 20 year break in the middle, about 75,000 miles
They sell a restore kit that includes a cleaner and an oil with good instructions ,I get at least 4 cleanings out of 1- $16.00 kit
I would not use any other type of oil.
The K&N filter allows better air flow when clean

Most people apply too much oil. They require a light coating, it shouldn't be dripping. If it's dripping then you'll just be restricting air and clogging up your inlet with oil. They should be oiled to the point that handling them leaves oil on your hands, but it shouldn't have any oil built up on the surface.

The K&N kits are good and easy to use, you can just apply an even coat with the spray bottle and you're done.