rust inside the engine oil filler tube
 

I recently picked up a s5 longblock with 130000 miles, when I unscrewed the oil filler cap I saw rust on the threading of both the cap and tube, I think the engine had overheating problems but I just hope coolant wasnt sitting inside. Can I pour and fill with atf or Marvel Mystery Oil to help maybe clean the engine. whats the best way to remove minor rust from inside an engine? im going to remove the exhaust manifold to take a deeper look, spark plugs are filled with carbon buildup, no orange rust

I wouldn't assume you have rust inside the engine just because of rust in the filler tube. Condensation tends fo build up in the filler tube area over time, especially if the engine is frequently used for short distance driving, preventing the engine from reaching higher temps that would evaporate the condensation. Sometimes you will even see a foamy emulsion of water and oil appear there. I suspect the rust in the tube is from that phenomena.

much appreciated

Had the same issue opened up my motors and the rotors were totally Fu%#ed & rusted :(