Dark oil is no biggie. That's just oil out of the combustion. The air leaks in the nipple are easy to fix. New tube or tighten nipple as said earlier in thread. Permatex Ultra Copper sealer works where the muffler halves come together (and on that nipple). First clean with alcohol, then apply a little bit. Tighten it and whipe off excess. Worked for me. The tank could have a leak in the stopper, fittings, or sliced tubing. Replace it all and see if the air in the supply line goes away. Your stopper may just be not tight enough. The hesitation is just like someone else said, your low end mix. That should be adjusted by an experienced pilot or you risk getting it too far out of wack. It will need a quarter to half turn to make it less rich (it's in the manual..see fuel mixture setting).

Joe