If you want to enjoy your rolling stock for a long time, you will have to service them once in a while. Locomotives need a drop of oil to ensure smooth running, and you can use it to prevent squeaky wheels on wagons. In the past, LGB supplied an oil pen (Art. No. 50019), but it’s not available anymore.
Massoth now offers a 12ml maintenance oil pen for € 9,95 / 12,99 USD (MSRP).