I know that this is rather old topic, but it is still actual.
I have a Stelvio 4V with 15k miles on the clock. I have the same issue with the loud gearbox.
When engine is cold everything is perfect sooth. Up and down shifting, first gear from neutral at stand still jumps in without even a click.
Situation changes when engine gets hotter. Hotter->Louder.
First gear jumps in with a big bang and up and down shifting with the noticeable bang.
However clutch is not dragging. On central stand, engine running, clutch disengaged, first gear in, wheel is not spinning at idle.
I've started from the clutch free play check, bled out the system, master cylinder check and lubrication of any necessary parts. I removed the secondary cylinder, pushrod. Lubricate as it was said in the manual.
Put it all back together and.... noting changed. So I've putt more effort to the clutch design. What I noticed is that this, single plate, clutch has a tendency to have very small drag. It can't be avoided in that kind of design. It is mostly related to vary small clutch play between engage and disengage position and construction of a central clutch spring.
Since the "abnormal" behaviour started to be noticed from around 2000 miles on the clock, after a numerous checks and after 6 oil changes I finally reach the conclusion, which agrees with the Guzzi service investigation.
This gearbox behave like that.
There is nothing to be done except to pick the right oil and learn how to ride.
I have to agree that Redline Hevy is much better than the Guzzi factory oil, but the best from all I've tested was 85W140 ENI ROTRA MP.
There is one simple explanation for the above mentioned problems, especially for the first gear bang. All this is because of the gearbox and clutch design. After few k miles the gearbox is already broke-in, so it gets a little loose. I the same time it starting to work with a minimum internal friction.
Guzzi gearbox have 3 shafts (standard motorcycle gearbox have only 2). One is the main shaft connected to the clutch plate. It has a fixed sprocket which is connected to fixed sprocket on the second shaft. When it's on neutral no other sprocket are spinning. That limits the internal friction to only 3 bearing and 2 sprockets. The sprockets friction is mainly coursed by a viscosity of a gearbox oil. Viscosity is lower at higher temperature.
Clutch itself have a minimal drag and a clutch plate have rather big diameter and weight. All that leads to rather high inertia of that moving parts.
So imagine that the clutch plate is spinning at 1400rpm and suddenly got stooped. You get the bang (like in all modern wet-clutch motorcycles).
When oil is cold there is no problem become the high viscosity of oil. Generally higher viscosity less bang. I'm using 85W140 ENI ROTRA MP oil and as far I've tested, it is the best oil for my gearbox. Oil tested at temps down to 32*F.
Pulling in the clutch for at least 15s before putting the first helps a lot. There is time to slow down the clutch plate. However, I've noticed that after a log time in traffic jam when engine had 257*F I had to wait 2 minutes to putt into first without the bang.
Factory oil is to thin in that temperature and there was no way to force the clutch plate to slow down. The same was with Redline Heavy.
I've done more than 12k miles using high viscosity oils and any abnormal wear wasn't noticed at all.
There is also one more trick that I use to up down shift without any sound. This is also the result of using this kind of a gerbox with dry clutch.
I noticed that the gerbox clunk more when I disengage the clutch fully. When I just pull the lever as far as half of an inch in the clutch starts to slip and that the point when you change the gear up. Plus throttle close-open bleep you can have a great results. It needs some practices but when You get it You will noticed the difference. It is not a gear change without the clutch. Clutch is half engaged eliminating the revert bounce of the slowing down gears due to the constant momentum applied.
Some say that applying a force on the gear change lever will decrees the bang. Generally it is not true on single clutch Guzzi. Don't do it, otherwise an increase shift drum wear can be noted.
I can describe in more detailed way, with pictures and stuff, what happens in the gerabox and why they clunk, if any of the forum user want an explanation.
Above mentioned methods was tested on 2 Stelvios and one Griso with the same results.
P.S. Sorry for my English.