thats the right reading on the factory gauge, as for overfilling a diesel it can be a bit risky if its too high, when you overfill the diesel engines the oil get up into the cylinder head and starts running on straight oil!!! when this happens the revs pick right up and sounds like bout 8k rpm + and keeps rising, along with mass smoke everywhere. i have seen this happen, luckily we managed to suffocate the engine before it blew. also i usualy get it warm before changing oil, it flows out easier and helps flush out any sludge laying around the engine.