...and maybe a 6-pack of beer. Have him change it out, then go play around on those hills and see if the locker is behaving as expected. Then, of course, let us know!

As for any potential damage. I don't know really...I doubt it but don't want to speculate.

 

local LR dealer said to buy twice as much and flush... $400
British Atlantic said to just drain and refill the residue of old should have no affect $200
Really don't think flush is needed and I am not trying to hurt the guy.

 

FINAL UPDATE: Fluid sorted... took LR branded fluid for lock rear diff, t case, and front diff to indy to change. Owner was very good and positive. Changed fluid, gave me what was left and ran another diagnostic saying he could find nothing wrong with diff locking motor and refunded cost of fluid. Drove home, no six pack but did go on test drive. ALL WORKED GREAT!!!!! Rear diff lock as expected and quick. All modes of Terrain worked as expected. Problem solved! Dokata T had it.

Do not know if all LR3 locking rear diff do not stay locked in reverse, but mine does not

NOTE: It had been the standard to fill front and rear diffs the exact amount specified. Now Land Rover says to full to bottom of fill port, I assume vehicle level. This is more than the exact amount specified originally. According to tech at British Atlantic, they need to change their YouTube video.

Thank you for all your time and help Dakota Traveler and houm_wa plus rest

 

...and we all got to learn something along the way.