BMW made 2 GSA models in 2013. The 90th anniversary twin cam, the last of them, and the first of the water cooled models. If you have a 90th anniversary twin cam with 1400 miles on it then color me jealous, that's a lovely motorcycle.
The final drive unit is serviceable just as it was in 2012,11 and 10. Both fill and drain plugs. It takes 180 ml of oil, put more in and you will have problems. BMW spec a 20000 kilometer service interval but I have always changed mine at 10000. As I said in my earlier post its not much oil and it does a lot of work.
It takes literally 15 min to change it and clean oil is good oil. Some folk just cant get their head around how little oil goes in there, say in comparison to your Gl1800 which takes approx 500 ml. I have seen 400 ml come out of a GSA FD just recently on a bike a mate bought 2nd hand. Started leaking oil profusely in the middle of a run and we drained it on the side of the road. Put in more oil than necessary and you will eventually blow the main oil seal. After that things can get messy.
Personally, if you like the bike then buy it, service everything including the brake fluid and ride it, they are sweet bikes. Then service the diff everytime you do the engine oil. There was a post relating to the twin cam models around here recently and if memory serves Dave has an R model of the same year with 280000 miles on it, perhaps you could have a yarn to him about it.
Here is a pic of my 2013 anniversary model, its my 5th GS and undoubtedly the best one. It has 178000 kilometers on it and functions as expected, has the original final drive and I've had no issues with it. In case you were wondering, I havent had a diff failure on any of them.
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