1. Breather Hole
2. Fill hole and maximum fluid height
3. Drain hole
Pretty simple. But drain the fluid through bolt 3 and replace bolt when draining is complete.
Fill the final drive through hole 2 up to the edge with hyphoid gear oil. That's the maximum amount of oil it needs.
Regarding the breather's purpose:
Without the breather, the final drive case would be a sealed enclosure. The gear oil and especially the air inside it expand as the drive heats during use and the internal pressure would rise significantly. Likewise it shrinks and the internal pressure drops when it cools. The vent allows the pressure to remain constant in either situation. Without it, seals designed to hold oil with no pressure differential would fail prematurely. I think synthetics have a lower Thermal Coefficient of Expansion, however the air in the case would still expand and pressurize when hot.
As for viscosity and synthetics:
It's mostly about temperature also. The manual says to use a thicker oil for higher ambient temperatures. This is likely because oils based on natural petroleum change viscosity in inverse proportion with temperature. The gear lash is set for the thickness the oil will film at when at operating temperature. Synthetics are more stable across their temperature range - they don't get as thick when cold, but they also don't get as thin when hot.
Correct me if I'm wrong, but I think the manual says that straight 90 weight is called for in 70F & up temps. I can't even find a non-synth straight 90W, and I believe a 90W synth oil will be even harder to find. While synthetics outperform old-school oils, I'd be sure the oil I would use films at an appropriate thickness for my gear lash and riding conditions - whether synthetic or not.
