> Rosin is a band-aid and isn't required if it's strung properly and friction is mitigated. You can use virtually any non-stretching string of a similar diameter and it won't slip.
I will give a poor analogy to the reason for the Rosin - poor, because technology has changed over the last 20 years, or so. But for us Luddites:
Those of us, who, back in the day, drove air-cooled VWs which we serviced ourselves for the most part, always used a wee bit of anti-seize on the spark-plugs to avoid head damage, and we adjusted torque accordingly. Most especially those of us who were *really cheap* and used un-plated plugs. Today, plugs are, for the most part, plated from the factory and anti-seize is not recommended by manufacturers any longer. Similarly back in the day VW also used lug bolts, and there was a 'whole thing' on anti-seize on the threads (not the bearing face) of the bolt. Again, not so much today due to improved technologies and plating processes.
Getting back to Rosin - the OP is starting with a vintage German radio - and as I have noted on occasion, the Germans never use one part where three-or-more will do better. So, restringing such a beast is one of more troublesome tasks in the vintage radio hobby. Even getting the chassis out of the case/cabinet is a massive PITA. So, the idea is to do it once and get another 50-75 years out of the effort. The things that mess up dial mechanisms:
a) Age: The pulleys wear, the springs (if any) wear, and the bearings wear such that the pulleys wobble.
b) Dust and airborne crud contaminate the system causing additional friction, or slippage.
c) And many hobbyists rely on their memory rather than looking for a diagram when re-stringing.
There are few absolute cures for a) - I have found that going to the next higher (or two) string gauges can help for two reasons: A greater bearing surface, so less tension is required, and therefore less wobble. And the string centerline is further from the edge of the pulley - again, less wobble. Replace the springs, of course.
Dust and crud can be cleaned - in fact a radio from a smoker environment will not see the display in our house until absolutely thoroughly cleaned. Most will be rejected prior to purchase in any case.
And, there are sources for stringing diagrams, even for many/most Euro radios. Further, the generic diagrams are useful to find similar configurations. I need to repeat HAND MATTERS! And may German radios tune from Right-to-Left - matters even more!
What the rosin does best is reduce the amount of tension required on the string, and together with the slightly larger diameter increase the friction without increasing wear. And if simple Dacron is used, that first application is well-and-truly life-time. The analogy that comes to mind is the fiber core of elevator ropes - as a reservoir for lubricants - the Dacron acts as a reservoir for the rosin and does not spread it all over the place as a sticky mess - a little goes a long way.
Yes, I keep high-tack PTFE-impregnated synthetic oil for bearings and such - it is emphatically non-conductive, and stays where-put, does not oxidize and does not turn to varnish even in warm environments. NEVER graphite as it is a conductor, and can 'track' even if by accident. Horrible stuff in an electronic environment, especially with tubes involved.
It is a process - and if done carefully, those next 50 years are a near certainty.