Best lubricant to use for maintenance on a Prusa I3 MK3?
Bearing and rod Lubricant
I use the Carlson's Gun Lube on mine and it has worked very well for over two years. It comes in a 30 ml syringe and stays on pretty well. It is a white grease that prevents rust and keeps dust and debris from bonding.
On packing your bearings...
Please consider my input below (sorry in advance for the windy bit to follow):
I used to be an oiler for a huge Machine Shop that had "white glove" standards. I was sent to coolant and oil tech seminars. I took care of lots multi-million dollar machines. So I know a little about oil & grease. A little red flag popped up in my mind about packing bearings.
Seeing posts about scored rods concerned me as I never have lubed my MK3s in 1.5 years of light to moderate use. It is running very smoothly now. I am getting ready to do maintenance tear down and lube job.
Linear bearings require a lighter grease/oil to promote circulation of the lubricant for light duty operation like in our Prusa printer bearings. I am not a fan of lithium grease as it tends to cake up/ get crusty. Many recommend Super-Lube. I had some questions so I contacted Super-Lube about the suitability for their grease + find out about it's NLGI ratings and their thoughts on packing linear bearings.
IMO opinion (which they confirmed) you should not completely pack your bearings. This can inhibit the bearings circulating in the races. They recommend packing about 85%. (I was thinking 75%)
Super-lube has an NLGI-000 rating which they say is like the viscosity of cooking oil. As bearings rotate friction causes the grease to heat up, liquify and circulate. Over packing is not ideal for circulation.
I can confirm that on my second printer (a non-prusa), which had drive screws that squealed loudly, super-lube stopped the squealing when even the fancy, ultra-expensive teflon lubricant I referred to above had no effect.