I am changing the oil in my Devilbus #247 air compressor today and forgot how high to add the oil. There is no dipstick or sight glass, but only a fill port that is on a 45 degree angle and has a plug that screws into it by hand. The book says "filled to first thread below Oil fill Port Opening". To me, that means the bottom thread of the Oil Opening. If I were to fill it to the top thread of the Oil Fill Port, the plug would then be screwing into oil. If only they would have shown a picture...