lets be real, is it going to go bad if it sits in an engine or hydraulic tank?

Simple answer is yes it can. Due to temperature variations air is drawn and expelled from the sump. In that air is moisture that can condense on the inside of the tank. The condensation builds up over time and can contaminate the system with water. I have seen more than one piece of excavation equipment have it's hydraulic pump destroyed over the years from a gradual build up of water in the hydraulic system. Another thing that happens to oil once removed from a sealed container is it can start to oxidize in the presence of oxygen found in the air. Lubricants also have a shelf life. Over time additives can start to drop out of suspension. Most lube manufacturers put shelf life at somewhere around 3-5 years.

Best practice is to have a UOA program in place. This way you can monitor water contamination and oxidation along with other contaminates.

No doubt about it; You can purchase breathers with dessicants for moisture removal from some filter manufacturers. Available online.

Well, it's all relative. If you have Group1 oil, it will oxidize much quicker than Group2 oil, but it will hold additives (mostly anti-foamant) dispersed in oil much longer than Group2 oils. Synthetics are entirely different animal, but they will last even longer due to slower rate of self-degradation than above mentioned oils. Additives separation is not that bad because they would be dispersed as soon as this engine is fired up or the hydraulic pump starts moving oil around, providing that there was no water in oil to hydrolize additives and as such precipitate them out.

Also consider that once corrosion has started in a spot in a part, the oil around it uses all of its anti-corrosive ability and the corrosion can spread.
I constantly battle this with people who put their harvesters away for the winter, or stop construction equipment between jobs. Very high corrosive damage on the next analysis.