What's causing the compressor to get oil in the air tanks?

looks like this one with just a different branding?

https://www.alltradetools.com/pdfs/new/archive/Powerbuilt-HotRod-5-Gal-Air-Compressor_647376_eng.pdf

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its because this compressor does not provide oil free air hence it should not be used to provide breathing air , but is perfectly fine for use with air powered tools , oil comes from crank case and piston blowby , ck your crankcase oil level every week add oil (30wt nondet) as nessacary to keep oil level up , compressor head needs rebuilt when you exceed 2 quarts a week usage

This problem is caused by either the starter capacitor or the in-tank check valve. Ez to check. With no air in tank, start compressor; if motor hums or has difficulty suspect capacitor. If motor starts, partially fill tank and unplug the power. Check for air leaking at the pressure switch or air coming out of the air inlet (remove the air filter). If air leaking, replace check valve. If no air leaking, again let out all the from the tank and connect power. Allow the compressor to fill and shut-off. After the compressor fills, again listen for air leaking from the pressure switch unloader valve. If air continues to leak from unloader, replace the check valve. Good luck

I don't know which model of air compressor you own, some models require no oil whatsoever. The important thing to remember is that you should never put oil inside the tank. While it probably won't damage the compressor and may, in fact, be good for air tools, oil does not belong in the tank. The tank itself should contain clean, dry, air ONLY. Depending on the job you're performing, oil and water in the air tank will ruin your hard work, especially if you are spray painting. The air compressor motor does require oil (unless, again, it's oilless), just fill it through the oil filler hole until it barely starts to leak out. You can change the oil every year if you use your air compressor professionally. Most people who use their compressor only occasionally never change their oil or even check the level. My compressor is a 20 year old Campbell Hausfield with the original oil. I don't recommend that, just stating the facts...I do religiously drain the water out of the tank with the drain valve, and I have replaced the drive belt. Once.

I would not worry. Drain the tank completely, and check the oil in the compressor, top off if necessary. Watch it for the first cycle.

There is no oil tank. The oil is in the crankcase of the compressor

you stated you replaced the filter.Was it the separator filter? the separator filter is located inside the separator tank. the separator tank is where you add oil. If this filter is bad it will cause oil and air to mix and get into the air. Also there is a 1/4 inch line that comes from the top of the tank . this line has an orffice in it If the orffice plugs oil can carry over into the air.

Dewalt claims that motor oil will cause carbon deposits to form on piston/cylinder head and on the inside of the discharge tube. I see carbon deposits on most all compressors that come in for repair. The carbon is not reason for repair. Usually forgot to add any oil causing rod failure. I believe that most compressor oils are just mineral oil. If the climate in your area is moderate, and you do not use the unit for 8 hours a day, motor oil will work just fine. The crank is steel and the piston rod is made of aluminum (no bearing shell). What makes the unit last longer is change the oil more often (holds very little oil). Good luck

It sounds like your fault could be worn rings on the compressor pistons causing slow pressure buildup and excessive oil in tank. Replacing rings, checking poppet valves for proper sealing and having cylinder bores honed should fix problem.

No additional moisture will accumulate over time. Some existing moisture may condense as the unit cools down, though. If you drain off the water at the end of the day, I see no good reason to completely drain the air.