A brushless motor used in RC is actually quite simple mechanically. There are only a few moving parts in the entire assembly. Those moving parts are limited to the rotor containing the permanent magnets and the bearings that support it.
When considering lubrication, the only area that could possibly require it is the bearings, right? To understand the answer to this question, we have to get a good understanding as to how the bearings are protected. The typical bearings found in an RC motor have a shield that helps protect the bearings from foreign object debris AKA dirt. The key takeaway from this is that the bearings are not water proof and can certainly allow liquids to pass by the shield.
Cleaning the Bearings
If you happen to try and clean the bearing with a product similar to WD-40, the oil can actually pass by the shield and mix with the factory bearing lubrication ultimately leading to possible damage to the bearing. To clean the bearing, my preference is to take a dry cloth and wipe any dirt away from the visible bearing surface. I do not like to use any chemicals.
Lubricating the Bearings
The quick answer to lubricating or not is that I recommend not lubricating the bearings. First off, it’s not as simple as just placing oil on to the bearings hoping to get by the shield of the bearing. Light oils may be able to make it easily past the shield of the bearing, but do not have the required lubrication characteristics. Light oils will more than likely not be able to withstand the concentrated heat and friction created within the bearing. The oil would burn off and subject the balls within the bearing to excessive frictional loads.
To correctly complete the process the shield must be removed, the balls (whole race) must be cleaned and then a proper bearing grease must be used prior to placing the shield back on to seal the deal.
Is this Process Worth the Effort
Here is another point to make. Some may say absolutely! However, I’d prefer to spend the price of a complete new set of bearings for the front and rear of my motor. It would be far less effort to replace the bearing set. I will also feel comfortable knowing that I have a fresh set (0 hours) of bearings.
One last point to be made. Out of all the brushless motors that I have ever used, only one of my motors are in need of new bearings. However, this was a motor that also lived it’s life upside down submerged under water when the boat flipped! Here’s the story. To put this in to a different perspective, the motor was significantly abused.