The tank is 15.5l? I never get more than 12l filled in so I guess the fuel lamp comes on when there's 3.5l left.
I get 16 km/l pretty consistently, riding as hard as I am able + in the mountains
My kawa CM mechanic also recommends semi-synthetic so that's what I put in. He claims it's better than fully synthetic in the hot / humid conditions here - I have no idea if that's true, just passing it on.
Make sure you keep the chain greased properly, I have had some issues with that recently - it seems like I have to wax it before any major trip. Oil is OK too but it leaves a huge mess all around the chain, so I use wax now. Putoline ceramic chain wax.
Oh right about the mountains up north and high revs in 2nd and 3rd gear - ride very carefully, I have seen many (many, many) accidents, especially on the 1095 from Pai - CM. There's bad visibility on a lot of those corners, and anything could be happening around the bend, from a truck flying downhill in your own lane to a sole stretch of sand on the road - there is currently exactly 1 10m stretch with a sand dune on the 1095. Because it's the only one, you really don't expect it.
Accidents on the Pai-CM road are cleared within a few hours - efficiency that comes from lots of practice!