back door getting stiff to open and close

The hinges on the Gen 4 have covers fitted which provides  protection from water ingess into the top of the hinge.  May be these could be modified to fit the Gen 3.  I have fitted internal covers which also help to stop water ingress.

This is the way to do it. On my gen 3 I drilled a 1mm hole in the central fixed part of the hinge and use a needle point grease gun with a 50/50 mix of moly grease and oil, Seal the job with a smear of wheel bearing grease and your hinges will last forever. Make sure you drill the hole in a position that it reachable by the grease gun by offering to the hinge first and its obvious where the right place is.