Waxes and vegetable oils are all very "right on" but not very practical in real life. I have fitted 12 internal doors in our house (only 2 more to go!) and have finished them all with Dulux trade polyurethane varnish, which stabilises and protects them against drying and shrinking or damp and swelling:
I believe you can only get this from Dulux decorator centres, which are great places to buy from because the staff are friendly, professional and helpful and if you get a loyalty card you get a further discount on their already good prices. The Decorator centres sell better quality paints and varnishes than you can buy retail because professionals don't want to have to put on lots of coats. From the little I know about paint formulation I can tell you that there is massive variation and cheap paint is all thickener and filler and very little opacifier.
Back to the varnish... this stuff is very easy to use and the satin version gives a nice subtle sheen just like wax. It is extremely forgiving of bad technique, once dry dribbles just disappear and brush marks don't show. Rub down before applying and put on two or three coats, rubbed down between for the best effect. Don't use Ronseal varnish, even their satin finish is horribly shiny and shows every tiny imperfection.
PS: A week after you've applied this you can go over it very gently with a fine sandpaper and remove the tiny bits of fluff and dust, which inevitable get onto sticky varnish and you will end up with a finish, which feels super smooth and pleasing to the touch... you could even wax it if you wanted.
