10W-40 or 15W-50 for F650GS??

[abuhawa]

Hello,I am a new motorrad in town. I have F650GS registered Dec 2009. Actually already own it since June. This is my first big bike. Got my full B after bought this bike. Now clock around 9500 km. I bought this from another owner.I just got the clutch replaced by Auto Bavaria (managed to claim warranty fortunately). The clutch plates slip even though it is less than 10,000 km! I have been using PETRONAS Syntium fully synthetic 10W-40 (Auto Bavaria accept it) since I got the bike. That what was recommended by BMW as explained in the manual.My check with Sunny Cycle mechanics, they claimed for Malaysian hot weather, for 650/800GS, should use MINERAL 15W-50 not 10W-40. 10W-40 is only for cool climate countries. 10W-40 will become too thin at high temperature and make the clutch slip (faster friction rate).When I told this to Auto Bavaria, they said no, should stick to 10W-40 if I want to maintain the warranty by BMW.I am in dilemma now. Which one to use.