Hi Ron,
Your air dryer should have captured most of the oil in the desiccant cartridge. Check if there’s any oil at the exhaust port of the dryer. With 350K miles on your compressor, you’re nearing its typical lifespan. Oil in the air system can damage all rubber components, not just O-rings.
For pricing, check with local heavy-duty parts stores. If you’re considering a rebuilt compressor, remember there will be a core charge, so you’ll incur shipping costs both ways. New compressors typically cost more but don’t require a core return. You can find the compressor details on a 2×2” tag riveted to the side, which should read something like TU-FLO 5xx (type) and 2xxxxxx (part number). It’s also a good idea to replace the governor while you’re at it—they’re inexpensive.