Oil in my coolant tank

Engine oil in that tank means you have a bad problem. No sealer or mechanic in a can is going to do much for that. The only repair is to pull the engine apart and find the problem. But check it real close as it could be transmission fluid from a leaking cooler in the radiator. Also, check the engine oil real close as coolant in the oil will kill that engine very fast. A common item on these is the lower intake manifold gasket.

Not good. Are you adding coolant and not seeing any leakage on the outside of the engine or elsewhere? That is not good either. I would do a couple things, Pressure test the cooling system. That will at least tell you if you have a coolant leak. Then have the cooling system flushed, that vehicle should run Dexcool, it is not green and adding conventional green can cause a nasty sludge that looks like oil. If you are in an area that is not going to freeze soon you could do the flush and only put in clear water. Then take it for a drive and see if it still shows oil.

Dexcool does not play well with other coolants. Flush the system. The intake manifold gaskets are known for failing due to the way the new engine design clamps the gaskets. Personally I would start by flushing the system, if you can find some the old Dawn machine dish detergent (not the hand wash stuff) it really does a number on the grease oil and deposits in the system. Or you can go with Prestone cleaner. Rinse it well then fill with water and see if the oil returns. If it does you should be able to tell for sure which it is. Then form a plan. If you are seeing leaks you will need intake gaskets anyway. So plan on intake gaskets, thermostat, replace the hoses and new coolant (go with DEXcool), possibly a water pump as well.
If the oil does return, and it is transmission fluid, add a radiator. If it returns and it is engine oil you will want to think a bit.