As Long as he holds a valid passport with a US visa in it, like you state, there are no out of routine things expected on this border crossing.
Land or Air don't matter if the documentation is all in order.
He can go back and forth via airports without issues.
Then he can also go via the land borders without any issues.
There are few exceptions, but most of the countries do not recognize a driving license as a valid travel document.
In case of North Macedonia, you need a valid biometric ID card or passport(source - Ministry of Foreign Affairs)
You should careful pack the oil, some airlines requires double layer.
3-4 liters should be OK for Germany, if you import just oil (ev. with few other cheap items).
The only airline I know that has restrictions on olive oil is Aeroflots, and only on flights from Greece to Russia (no such restriction on oil on other flights).
After actually crossing the border yesterday, I can now answer myself.
With a posted waiting time of only 3 minutes, we decided to use the border crossing from Vyšné Nemecké to Uzhhorod. There were only about 20 cars in front of us when we arrived at the exit checkpoint at the Slovakian side of the border, but almost no progress. We soon realized that ...
As you are not the owner of the car, you will need a letter of authorization from the owner. Please contact the rental company for it and eypect to pay an additional fee for it.
Which border checkpoint to take depends on your starting point and your destination. The major one is Kulata - Promachonas.