Damping grease is typically used for a number of applications including damping vibrations. This stuff is literally like magic on quads of all sizes. Smear it mainly between where the frame arms connect to the frame body and you'll see drastically improved vibration management on pretty much any quad. You can put this stuff between every joint on the frame but it might get a bit messy so we mainly just use it between the arms and main body.
Note that this stuff is EXTREMELY viscous and sticky. If you get it on your hands, it doesn't really wash off. You just need to let it naturally rub off over a few hours to a day.
With respect to flight performance, adding this damping grease between your frame joints can make a total mess of a craft actually fly well, and a well flying craft fly as smooth as silk in the sky without touching anything in software. We've yet to come across a quad that doesn't improve by adding this grease but sometimes the benefit is more along the lines of reliability across environments rather than super obvious flight improvement.
For the general application of multirotors, the heavier the damping grease is, the better. The leading option for us has been something called 'PG-44a'. Good stuff. Expensive, but good. We tried to work with the company to create something specifically for multirotors but they refused. Furthermore, stocking their product is so expensive we literally cannot afford it so we went hunting far and wide for damping grease. This is the best stuff we've found and it matches the performance of PG-44a in every result we tested it against. The only real difference is the liquid texture. Both greases are 'HELLA' sticky but PG-44a has a bit of a granular texture while our stuff is just straight hella sticky. The best part is that you probably won't ever need more than a single jar of this stuff and the cost per gram is a fraction of PG-44a.