1. Change the oil to a GL4 approved GB oil. Go for a fully synthetic. Im using Castrol VMX 80. It was good at first but being a mineral has lost its effectiveness.
Im going Valvoline Duragear. Ensure you use a GL4 and not GL5 as it contains additives like sulphur that eat yellow metals like your copper synchros over time.
2. The easiest way is to get sealant and just line the whole light where the housing and the case comes together, assuming there is a gap in the original sealant somewhere.