It depends. This is where it gets more complex, or even complicated, but I'll try to keep it simple.
Reading my notes (I've kept detailed notebooks on test data with films, exposure, papers and printing since I took up darkroom work in the 1960s), J Christian Lootens, a popular (and prolific) author of darkroom books during the '50s and '60s, in his book 'Photographic Enlarging and Print Quality' (I have two copies dated 1958 and 1960) recommended a 'mix' of 10% benzo (so 10 grams to one liter) to be added to the print developer in 25 ml shots. He also advised that developing old papers is best restricted to one minute or less and ideally 45 seconds. (You can increase the concentration of your print developer accordingly.)
I'm currently using up some old (1990s) Ilford Multigrade III 8x10 paper I've had stored (refrigerated) for about ten years. Bought it on Ebay when the going was good. MIII is a developer incorporated paper and many claim it doesn't age well, but so far I've had no problems with it other than very slight fogging in much older (1990s) MIII. Oriental Seagull will probably give even better results.
Lootens also recommended using potassium bromide (again in 10% solution) in the print developer for the same purpose, but this I found can alter the color of finished prints in certain papers, so I tend to use it much less.
Another tip from him was to use sodium carbonate in the developer (again in 10% solution) to deepen blacks, zip up old developer and clean up the highlights.
My personal choice of added chemicals for my paper developer runs as follows - benzo often, sodium carbonate sometimes, and pot brom now and then, if I find I have very old paper in need of all the help I can give it and I don't mind a slight change in contrast to warmer or even towards a green-brown tone.
Play with the amounts you put in (I work in 20 ml increments) and see what you get.
All this works for me, but it varies according to the age and storage conditions of your paper. For me much of the fun with old papers is in experimenting, but you must keep in mind you may not get the same results with every sheet you expose and process.
(Added later) Other darkroom 'fanatics' may have more suggestions or comments re all this. I look forward to reading what they have to say. Please post.
