Have you tried lucas oil stabilizer? It's a bit stickier, and helps the syncros grab without wearing out so much

All I'll say is that you should do some research on the BITOG forums about Lucas products and see what you think.  It's possible to increase the viscosity of your oil and quiet noises or dampen the feel of things while still having a negative impact on the performance of the oil.

You've seen the Lucas product that is advertised in the parts stores with the gears that you can turn to see how much better it "flows" to the top of the gears?  That product is essentially pure petroleum and there used to be a video on BITOG that showed how it performed in an actual differential turning at normal speeds.  It was so thick that it actually foamed which is just about the worst thing a lubricant can do because it puts air pockets in the fluid.

Take a look and see what you think.  There are some Lucas supporters on the forums and the support, or lack there of, is likely product specific.