[Tom, F&L GoR]

At higher RPMs the difference between the synchro ring and gear is greater, so it takes longer for them to come to the same speed and allow you to engage. In an old 833 4-speed, I roughed up the bronze cone surfaces and it engaged much more quickly, until, of course, it wore smooth again.

Consequently, a thinner fluid should provide more friction on the cone surfaces to allow quicker engagement, so you are on track there. I don't think that the amount of bearing surface comes into play, simply the inertia of the metal spinning parts. Typical auto transmission fluid is 8 cSt, or an SAE 20.

With a thinner fluid you will probably hear "neutral gear rattle" or more. One successful fix has been to modify the clutch pedal ratio to get more clutch departure. Any slight drag on the clutch during shifting will make shifting more difficult; changing the ratio is like pushing the pedal down farther without having to push it farther...