How critical is the heavier weight for the 690? I see 10W-60 & 10W-50 listed. What keeps me from using the 5W-40?
I've heard that it's because of the silver bearings that 10W60 is required:
Note that there's some indication that KTM has used silver bearings in the new LC4 motor. Using silver, instead of lead, provides greater strength than any other bearing material, and it also resists corrosion better and is more thermally conductive. Silver bearings are more typically found in heavy duty marine diesels or jet engines. Unfortunately, the detergents in automotive and motorcycle motor oils (primarily ZDDP) will attack and eventually degrade silver unless they are specially stabilized. This may be the reason behind KTM's strong endorsement of the Motorex Cross Power 4T 10W-60."
I'm not sure I really understand why10W60 & silver bearings are connected, but that's what KTM claims.
Some KTM forums thought that the oil was low detergent, low ZDDP, or otherwise stabilized to work with silver-plated bearings.
Here is an interesting oil analysis of the 10W60 that KTM reccomends:
Brand New Motorex 1-W60 Analysis:
Right out of the bottle it doesn't measure as a 60-weight; it looks more like a 15W-50, actually.