What do you use to polish wood plinth?

Agree with the others. Wood can be finished in many ways with many different materials. Oil? Shellac? Varnish? Wax? Polyurethane? Matte? Satin? Gloss?

Attempting to refinish one part of an older wood piece without knowing anything about the original finish is guesswork. With so many possibilities, most guesses will be wrong and some may cause problems that can't be fixed.

Your results make it evident that lemon oil is one of many possible wrong guesses, so stop applying it. Ask SOTA what the original finish was. If they can't help, ask a local wood finishing expert. They need to be local so they can actually see and touch the wood. I know everything on the internet is true, but no one can deduce your turntable's wood finish from a distance. ;-)

I've used Old English on my Denon and it looks great.

I would agree with Elizabeth that someone tried to fix a marking and ruined the topcoat in the process. One solution is to remove everything from the plinth so that you are left with just the wood to deal with. Take 0000 steel wool and remove the balance of the to topcoat to an even dull finish. Then remove all dust with tack cloth and re-apply a topcoat finish of your choosing. Try to pick a satin or a semi-gloss to help conceal any imperfections that may exist. Apply as many coats as you wish till you reach the look you are most happy with. Just repeat the steel wool and tack cloth between each coat. I would say that a spray poly would be the easiest to work with or you may want to use a waterbased clear finish. But from your description, I think refinishing is the inevidable solution. Trying to patch it in will create more frustration and may cause further damage, requiring restaining and then refinishing. You can find many woodworking sites that will walk you thru these steps to make it seem less daunting.

If Howard's Feed-N-Wax didn't work I would just adhere a nice Hello Kitty sticker to the spot and call it a day.