I have a lot of old (80s grey era and 90s 9v) trains. Some of the wheels squeak and many have quite a bit of friction. Sometimes this friction causes the locomotive to detach from the rollingstock since the friction of the wheels on long trains is greater than the magnetic force between the couplers.
I searched this site and the closest match I could find is: How to lubricate the 4.5v train motor for set 7722? It mentions using 'plastic safe oil' but does not specify a brand. This warning is echoed across the model train internet and some (non-Lego) discussions have recommended using a full-synthetic motor oil applied via a needle oiler.
The linked question is specific to the motor and gears that are likely to be designed to be lubricated vs wheel axles that are not. I was going to use 3in1 oil but the consensus seems to be that this is a bad idea because it is a petroleum derivative.
Does anyone have any first-hand experience of a 'plastic safe' oil that they have used on their Lego? I would think any oil would work to lubricate the axles but I want to be sure it will not damage the plastic over time.