Some folks say to use SAE 30W non-detergent engine oil, as recommended by Piaggio many years ago. Others say to use SAE 75W/85 gear oil with API GL4 (NOT GL-5) specifications, as recommended by Piaggio for the last 25 or so years. Some say that the two are the same.
The two are not the same. The viscosity of the two is about the same at 0 degrees, but the 75W/85 gear oil will hold its viscosity with increased temperatures, while the 30W will thin to about SAE 20 at operating temps. Additionally, gear oil is formulated specifically for gear train usage. Confusion often arises because SAE uses a different viscosity scale for gear oil and motor oil, so oils of the same actual physical viscosity will have very different numerical ratings.
NOTE: Some oil companies have begun to show the viscosity of their gear oil using engine oil SAE ratings. Thus, you will find SAE 10W/40 GEAR OILS, which are effectively SAE 75W/85 viscosity.
Why did Piaggio change the spec from SAE 30W ND to SAE 75W/85 GL-4? Not because of any significant change in gear box design, but because the better performing gear oil was not generally available for the earlier model Vespas. When a lubricant more suitable for gear box applications became a standard product, Piaggio changed the spec to take advantage of the improved properties.
A pretty good presentation on using gear oil versus engine oil can be found here:
Gear oil is specifically formulated to offer better lubricating protection to gears than engine oil.
Why NOT GL-5? Here's the basic GL-4 versus GL-5 gear oil story:
As a general rule, GL-5's incompatibility for systems with copper or copper alloy components is due to the extreme pressure (EP) wear additives. To give the gear oil its EP rating, it's traditionally been easiest to add a sulfur based friction modifier like MoS2 (molybdenum disulphide) but the broken down sulfur left in the oil can cause copper based metal corrosion. GL-5 often differs from GL-4 by the amount of these additive chemicals that are mixed into the oil (often twice as much or more in GL-5), and thus the undesirable corrosive effect.
To get the GL-4 rating, the oil has to pass the ASTM D-130 test. This test determines how reactive the EP additives are with a polished copper strip. During the test, the strip is also subjected to heating to simulate the running conditions in the gear box.
To obtain GL-5 certification, the product does not have to pass the ASTM D-130 test. Thus, two "warning flags" with straight GL-5 rated gear oil: (1) GL-5 can have higher levels of the corrosive (EP) additives, and (2) GL-5 is not tested to determine if it is corrosive to copper containing metals.
Motul, for example, sells a gear oil with GL-4/GL-5 rating. Their spec sheet says "Suitable for any type of seal and yellow material used in gearboxes design", which infers that the EP additives they use are not corrosive beyond ASTM D-130 standards, which would tend to say that the specific EP additives they use are not of the same corrosive nature as a typical straight GL-5 formulation of additives.
In summary, and in descending order of protective level:
GL-4 gear oil is the optimal for our shifty gear boxes. It is a hypoid gear lube specifically formulated for the needs of gear trains, and its EP additives do not exceed the corrosion limits of ASTM D-130. 75W/85 gear oil will hold it's viscosity at operating temps.
GL-4/GL-5 gear oils should also meet the corrosion limits of ASTM D-130, and it's higher EP rating has no real impact in a shifty Vespa.
SAE 30W non detergent engine oil is an acceptable oil. It is not specifically formulated for gear trains, so it will not afford some of the protections of hypoid gear oil, but has no corrosive EP additives. Also, 30W engine oil will thin to an SAE 20 or less viscosity at operating temps.
GL-5 gear oil is discouraged as it usually has additives that are mildly corrosive to copper containing metals, and no testing has been done to determine if the corrosive level is acceptable for copper containing metals. There are a variety of copper containing components lubed by the gear oil in our shifties.
So, three of the four oils should work, with varying degrees of lubrication protection. Use what tickles your fancy. To actually "measure" the differences in protection (gear oil vs engine oil) or corrosion (GL-4 vs GL-5) you would have to do spectrographic analyses. Much easier to follow Piaggio's current recommendation: SAE 75W/85 gear oil with API GL4 Rating. They have already done the "science" part.