The problem here is the the lower viscosity and lack of sulphur in the synthetic Redline oil. It's just too slippery for the synchros to work properly (especially worn synchros).
Synchros work by using friction to either speed up or slow down two intermeshing and rotating devices. Modern manual transmissions don't 'shift' gears, per se (except reverse) - gears are constantly in mesh. When you shift you are 'locking' a gear to a shaft, rather than letting it freewheel. Remember that modern gear boxes are in constant mesh. The gears are never pulled apart only locked or unlocked to shafts, hence the gear teeth can be helically cut at an angle to ensure quite operation. It's straight cut gears that give reverse that distinctive sound.
Alfas use Porsche designed synchros, which are completely unlike the traditional brass cone style found on most other manual transmission vehicles. The Porsche style rings are moly coated, beveled split rings that have an interior mechanism that expands the ring (to create greater force) as the sliding sleeve first contacts the synchro. The rotating, sliding sleeve is moved fore and aft into gear engagement by the shift fork. As the expanding ring starts griping the rotating sleeve (which 'floats' on a three prong yoke attached to the output shaft between 1st and 2nd driven gears). The driving gears being fixed on the countershaft. The driven gears freewheel when not engaged. There are small teeth on the face of the driven gear as well on each side of the sliding sleeve. Ideal these are spinning at the same speed when the meet and 'lock' the gear to the shaft (via the sleeve and fixed yoke.
The issue with Porsche style synchros is the the bevel facing (moly coated) starts to wear down, as does the inner surface of the sliding sleeve. The growing tolerance means the sleeve gets to the mating teeth before the gear gets a chance to come up to speed. The buzzing sound when you crunch the gears is the teeth riding against one another.
1st and 2nd share a sliding sleeve (center sleeve in pic below). 3rd and 4th use the same parts and set up. 5th also uses the same set up but the back slide of the sleeve is unused. A quick and dirty refresh of crunchy 2nd gear Alfa box that's fine otherwise (assuming it the synching teeth on 2nd gear are half decent) is to harvest the 5th gear sleeve and synchro ring. The synchro rings can be flipped around to present a less worn bevel to the sliding sleeve (on any forward gear). You then take the old 1/2 sleeve and use that for 5th, with the 1st gear side towards 5th and the bum old 2nd gear side now just no work at all.
In the illustration below the front of the box is on the left. 4th gear is the first gear on the left, on the input shaft - but the curved gear teeth are what drive the countershaft for the other gears. 4th gear is a direct drive gear - when you shift the 3/4 sliding sleeve forward (by pulling the gear lever backwards) you are locking the input shat to the output shaft and in essence making the crankshaft, input shaft, output shaft and driveshaft one long shaft rotating at engine speed. 5th/reverse is the the sleeve in the pic that has a really visible shift fork. The 3/4 and 1/2 foals are located on the right side of the gear case and the tips are just visible under the top case thru bolt holes. The straight cut gears on the counter and output shafts are reverse (plus a movable idler gear that is hidden below 5th in this photo). Hope this explanation makes sense.
As an aside to this particular issue, never overlook the dumb stuff - could a scrunched up floor mat be preventing full clutch disengagement?
