... different folks, different strokes. I read several locksmithing/lock picking books when I was younger and they all seemed to root for graphite. Reason being, you don't want something in there that will collect and coagulate dust particles into bigger pieces. It depends on the lock's environment, I suppose.

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'82 240D 224K miles manual transmission
mods: wooden 4by4 bumper, EGR delete and older EX manifold without EGR port, glass pack muffler (cheapest replacement muffler), rebuilt bosch injectors with Monark nozzles

working on: aux electric fuel pump, coolant/fuel heat exchanger/filter head, afterglow, low oil pressure buzzer/LED

The thing is, I don't know if I believe the locksmith. I've had terrific results from graphite. I know that on a long enough timeline all locks will fail: dirt and dust builds up in the tumblers, and it seems like a liquid would just coagulate dust particles into bigger particles, whereas graphite would not. Again, no expert, but graphite seems to work well.

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1984 300D

How expensive is it to replace the tumbler?

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1982 240
460,000 miles total
140,000 on Rebuild

I replaced my tumbler and drivers side door lock from junkyard parts, which was very cheap. If you go through the dealership and get a new tumbler and locks: much more expensive. I've heard 3 to 4 hundred dollars. Every so often the yard will get a car in with the key in the ignition. If you can pop it out with the tumbler (and the rest of the locks(trunk and doors)) and fit your car with the scavenged parts, you are back in the game.

It's just the same problem over and over: wait for the right parts at the salvage yard, or get it fixed now with the new tumbler from the dealership. If money isn't an issue, go through the dealership. If it is, find a workaround and wait for the parts at the salvage yard....

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1984 300D

A new cut-for-your-VIN tumbler and key from the dealer is around $100, depending on the dealer.

For the OP: have you tried "chilling" the tumbler with a steady flow of compressed air from one of those "dust-off" aerosol cans? It may be a middle ground between doing nothing and squirting oils, using sanders, etc.

The ignition lock mechanism costs more than the tumbler, but is not a dealer part and can be sourced from a JY. But you're possibly replacing one decades old part with another.

I'm a fan of the dealer-route for the key; maybe willing to risk a JY part for the other, depending on budget and condition of donor car.

I'll nominate my tumbler and lock for most modified by the PO: he removed all the guts from the tumbler so it could be turned with any flat object (e.g. a thumbnail) and then wedged a woodscrew into the steering lock so the ignition switch was more "switch" and less "lock".

For bonus points he then loc-tited the steering wheel in place, since to torque the hex screw you're supposed to have an ignition lock.

The ignition lock may not be dealer, but can be bought aftermarket through any number of sources including Fastlane.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)