Liqui Moly oil flush and Ceratec are the main additives I have used in my old TDI. If I had any oil leaks that were beyond economic repair, I would also used Motor Oil Saver stop leak additive.
If you are going to use oil additives, it is a good idea to stick to the larger engine oil companies and steer well clear of the back street snake oil mixers.
Liqui Moly MoS2 for the '25 & '70
Liqui Moly MoS2 plus Motor Oil Saver for the '37 (she's thirsty)
Used Ceretec once in the '70, but Liqui Moly advised it's formulated for newer engines.