Drain all the oil and check what comes out. If it is black, then it may have been "cooked" by a high heat problem or could have been run way too long between changes. Feel the old oil for any grit. If you feel "chunks", try to identify them. If they crush between your fingernails, they are most likely sludge or carbon. If they are hard, run a magnet through the oil and see how many you come up with. If they don't stick to the magnet but still appear to be metal, then you may be looking at aluminum casting pieces. If you see tiny metal flakes, a certain amount is normal wear and tear. Add the right kind of oil and move to the filtration system.
Remove the top caps to the moisture separator and the filter cartridge holder. The moisture separator is probably going to be a bit gunky so you can use something like Dawn dish washing liquid or Simple Green to help clean it out. You can run clean fresh water through it and out the drain valve to rinse it out. Inspect the separator for any signs of cracks or corrosion. Change out the O-ring, put a light film of silicone on the threads and put the top back on. Move to the filter holder and clean it out the same way. There is a spring sitting in the bottom so pull it out and inspect it. If it has been sitting a while, it may be rusted through. If so, get a replacement. If not, after you have inspected the internal walls as before, drop it in and close the top (with no filter cartridge).
Now, go through the normal startup routine (check oil level, open drain valves, open the fill hose valve, etc.) and get it going. Air should start blowing out of the drain valves. Shut the moisture separator drain valve first then the filter tower drain valve. Make sure they are actually closed but don't crank down on them since the seat is a fairly hard plastic part and being an animal with them can break them.
Watch the gauge as the pressure increases. It should steadily climb until the pressure maintaining valve opens and lets air out through the fill valve. Mark the pressure it opens and then shut the valve on the fill hose. Now keep going until the relief valve (on the side hear the top of the moisture separator) lifts and note the pressure. Turn off the motor and watch the pressure while listening for leaks. If the pressure stays steady and you hear no leaks, you are in fat city! If you hear leaks, see if you can identify where they are. Fitting leaks are pretty easy to find by spraying some soapy water and watch where bubbles grow. And don't worry about the soap. Compressors need a good bath from time to time to clean off gunk. Mark the leaking fittings but don't tighten them under pressure. If you have leaks anywhere on the compressor (like between the heads and cylinders) then it may be time for a valve job. After you have marked all the leaks, drain the pressure and inspect/tighten them and run the leak test again. When done to your satisfaction, put in a new inlet and outlet filter cartridge and start pumping.
I could give you a ton of "what if" scenarios, but it is better if you just read through the manual and address any matters as you run into them.
