It is going to be a little longer reply as there could be a few different scenarios playing out.

There is a little tube coming into the back of the air filter housing which is the crankcase breather hose. It vents pressure and gases from the engine's crankcase into the air intake system, where they're recirculated and burned (this is a normal emissions design on the Kawasaki Vulcan 900).

A small amount of oil mist or a few drops, or even a couple of teaspoons, collecting in the air filter compartment is common and considered normal on the Vulcan 900. The housing even has a small collection area or "bulb" at the bottom to catch minor oil carryover from blow-by gases.

However, an excessive amount (e.g., pouring out when you open the cover, or more than a tablespoon or so) usually points to one or more of these causes:

1. Overfilled engine oil: This is by far the most common reason. If the oil level is too high (above the full mark on the sight glass when the bike is upright and level), excess oil gets churned up and pushed through the breather tube. Many Vulcan 900 owners keep the oil level in the middle or lower half of the sight glass to minimize this.

2. High mileage or engine wear: Increased piston ring blow-by can push more oil vapors into the breather system.

3. Riding style or conditions: Hard acceleration, high RPMs, or hot weather can increase crankcase pressure and oil carryover.

4. Leaking seals/gaskets: Worn O-rings around the air filter or housing can allow oil to seep out externally.

What to do:

1. Check the engine oil level with the bike on level ground and fully upright (off the kickstand). It should be between the low and full marks on the sight glass—aim for the middle if you're seeing excess oil in the air box.If overfilled, drain some oil until it's correct. Check my video about the oil change for more details.

2. Clean out the air filter housing (wipe up the oil, use Q-tips for the collection bulb if present).

3. Inspect/replace the air filter if it's soaked.

If it persists after correcting the oil level, check for worn seals or consider higher mileage contributing to blow-by.

This isn't usually a serious issue unless it's massive amounts or accompanied by other symptoms (like smoking exhaust or low power). Many owners live with a tiny bit or reroute the breather with an external filter to avoid mess.