Run compressor first to warm up oil and bring up any particulate into the oil. Will flow faster and do a better job. Remove air filter to have room to use funnel.
In reality, the oil in a an oiled air compressor should really be changed once every 10 years and with 100% synthetic. The oiled air compressor really never runs hot enough to break down the compressor oils as any dirt or water that makes it into the engine should be readily absorbed by the oil itself. However, the DIY job is very simple and only takes 20 minutes at best.