It sounds like Wayne has given you a very nice base that you would like to add depth to. No, I'm afraid since it has been sealed already the tung oil would not be able to penetrate the pores in the wood. It would then be a superficial coating that is just problems.

There are two things I can think of that can help you out at this point. You can continue with a few more coats of Minwax, but it would really be best to remove the drivers for this so you dont get a buildup around the drivers. This might be visible much like painting around a door knob.

Minwax makes a wipe on poly that goes on in lighter coats. Just follow the directions on the can. I recommend this procedure since you could leave the drivers in the cab. You can still rub out between coats with 0000 steel wool as long as you do not use a water based product. For water based products you have to use a fine paper such as 400 or 600 3M trimite or similar.

For added depth and luster to your finish. Use a gloss wipe on poly.
rub down between each coat since polyurethanes rely on a mechanical bond and have no burn in. 4 coats should do you, but since I dont use this product you will have to rely on the directions and how it looks as you progress.

Now that you have your finish build up with gloss. rub down your last top coat with 400 paper, then lightly steel wool, and paste wax. It's ok to use steel wool after the last coat on water base.

You will notice that the final rubbing will bring down the luster a LOT! make sure you judiciously rub with the grain and with even pressure.
When you apply the paste wax you will have the most beautiful sheen and luster imaginable.

It would be best to stay with the Minwax products since that is your base. A poly, wipe on poly, or wipe on varnish will should have no compatibility problems. A oil modified varnish will age gracefully with an amber tone. solvent based poly will tone out a little with age, while the water based products ad little or no life but offer a very resilient water white clear coat.

Bill