Working fine for years is great, but an oil burner requires yearly checks to make sure everything is still set right. We are burning a liquid instead of a gas so things like oil filter , nozzle, electrode settings and pump pressure are critical to maintain. Just like car maintenance. This requires certain tools to make any adjustments. CO kit for air adjustment, smoke test for for proper burn, pressure gauge for pump pressure and access to correct nozzle settings depending on the oil burner.
I am honestly not trying to avoid answering your question, but to make any changes and perform any checks would require the proper tools.