My 2013 GT hit 21,000 miles but I ended up selling it before a new chain was due.
I changed the plastic chain rail at 15k, from the original #1 to #2 (of four, changed every 15k). Easy enough work. I replaced the gasket and used MOTUL Gear 300 oil (75W90).
My notes from that time are pasted below, hopefully helpful ....
CHAIN, CHAIN OIL AND CHAIN RAILS
The four chain rails, changed every 15,000 miles:
33 35 8 533 983 CHAIN RAIL - STAGE 1 0.18* 1 * $26.29
33 35 8 533 984 CHAIN RAIL - STAGE 2 0.20* 1 * $26.29
33 35 8 533 985 CHAIN RAIL - STAGE 3 0.20* 1 * $26.29
33 35 8 543 854 CHAIN RAIL - STAGE 4 0.24* 1 * $26.29
CHAIN: BMW # 33838544675
SHAFT SEAL: BMW # 33358532912 (gray ring behind cover, $35 with tax).
GASKET: BMW # 33358540920
CHAIN LOCK: BMW # 27727720938
CHAIN OIL: 75W90 500 ml. MOTUL Gear 300 1-liter 75W90, $21. Need Permatex Blue Gasket Maker, $6.99, before opening the case.
TORQUE DRAIN AND FILL PLUGS: 14.75 ft. lbs.
TORQUE FOR SWINGARM COVER BOLTS: 9 ft. lbs.
FROM THE FORUM: “As for the chainset, at about 15,000 miles the first adjuster can be installed. This will be done every 15,000 as there are 4 sizes of adjusters. At about 60,000 miles the chain and sprockets are replaced (and you go back to chain rail #1). I did my first chain adjuster last summer used Gasket Blue to seal up the castings, no leaks.”
FROM THE FORUM: “To check the chain, remove the black oval BMW plate on the swingarm. There is an arrow. Lift up the chain with a screwdriver. If the chain lifts above the arrow (½ inch) it is time for a new chain rail.”