Primed? You want to stain/clear it or paint it? Honestly doesn't really matter... anything will work, even untreated (unless its sitting in a wet area or dirt).
If its primed then oil won't do anything other than make a mess. Get a small can of exterior paint (oil or latex) and put a few coats on lower portion of the workbench legs.
Thanks for the suggestions. I was thinking of using an oil like teak or boiled linseed. Something that will protect it but not make it slick.
Go out to the garage. Find whatever clear(ish) finish you have. Slap it liberally on the bench. Top and bottom. Call it good.
In the event you don't have any clear(ish) finishes, head down to your local paint monger. Find whatever clear(ish) finish that is inexpensive and easy to apply. Buy it. Head home, slap it liberally on the bench. Top and bottom. Call it good.
Do NOT sand the workbench top past 120 grit. I wouldn't even sand it past 80 grit. The finer the sanding, the smoother, and thus, slicker, the top will be.
3 days after you put the finish on, go out to the bench carrying your coffee. Slosh the coffee around so some drips down the sides of the cup. Set the cup down on the bench. Find a couple of condiment packets. Set them down on the bench. Smash the packets with the coffee cup. Get a greasy rag and play with it for a while, then play patty-cake on your bench.
Go back inside. Wash your hands. Finish your coffee. Clean the bench next day. Now it's broken in and you won't obsessively try to keep it pristine.
It's a workbench, not the table in a formal dining room.
I would not put a urethane finish or varnish on it if it was mine. I have just finished up two work benches, and I put BLO (boiled linseed oil) on the maple one. It soaked in pretty fast, and it does smell like mentioned above. Several coats is the way to go. anything like urethane that does not penetrate the wood will just crack and come off over time.
What kind of work will you mainly be doing on it? Lots of water based liquids, building furniture, oils and/or fuels dripping, heavy impact forces, etc? Depending on what you will be using it for you might be better off just laying a thin sheet of masonite, plywood, etc. over the top and leaving it as is. Do whatever you want to the top without worry, because you just change out the surface sheet to a new one whenever it wears out, gets beaten up, cut up from a utility knife, covered in many layers of spray paint, etc.
In as far as oils, I just use regular mineral oil from the drug store for all my wood tool handles. It works great and it's cheap. That's the same oil most people use for wood cutting boards in the kitchen to give you an idea of what the surface would be like. I get that nice velvety feel without any smell, stickiness, etc. on my tool handles. Sometimes I warm up the wood handle first with a propane torch or a heat gun to help aid absorption and reduce the wet time, but it is usually absorbed pretty completely within a day or two.
Just don't put too much of any oil on the end grain of the lumber at one time or it might crack the ends a little bit from the oil absorbing and swelling the wood too fast.
I don't have any experience with oiling an actual workbench, so I don't know much about how well any oiled surface would hold up to something like gasoline drips or a spilled solvent for instance.