First of all you've bought a manual - either Clymer or Haynes, right? This book will save you thousands in maintenance and repair bills over the life of the bike.
The oil thing has been debated and beaten to death, I use the BMW oil mostly because the 85 K-100 is prone to sprag clutch lockups... the initial design has fewer oil holes, (problem solved with later models). When this clutch locks up, you can only pray that you can get it unstuck with aftermarket oil additives. The possible reasons for it getting stuck include using the wrong oil
, previous owners starting the bike in cold weather and then turning it off creating a situation where moisture gathers, but is not expelled and so forth.
I wasted countless hours fixing mine because the PO did both of the above.
Your bike and your time - personally I'd rather pay a bit more for the oil and be riding than hunkered over a ripped apart engine trying to spray solvents into tight gaps.
You do not need to remove the fuel tank to R&R the antifreeze, there should be enough slack in the fuel lines to permit you to lift the tank up a few inches to access the fill cap - for sure here use the right antifreeze solution.
Finally, you know about the other maintenance items like the input and output spline lubes, brake system flush right? This link will prove to be useful reading...