Here are some tips I picked up while figuring out my too much oil issue.
First, something that can hurt the spark plugs is running the engine rich too much. This happens when you have the choke open to warm up the bike, so there’s no oxygen getting in the engine.
For a Sportster what you should try is rather than open the clutch, turn the gas 3 times before starting the bike, and once the bike is started keep the engine revved to warm it up. To make it easier, you can start with the choke and then slowly push it in as you keep the revs on.
Now a quick tip for the air filter. Something that can apparently cause oil to flood into the air filter is riding at a high gear with low revs. So when you’re riding try and wait a bit longer before going up a gear.
Hope this helps, now all there’s left to do is just keep the rubber on the road!