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Gearbox Oil

The Haynes manual states to use SAE 30 motor oil but the Vespa owners manual states SAE 80W oil API GL4. Take care because although the numbers seem different they both refer to the same thing! Motor oil and gear box oil both have SAE numbers but the numbers mean different things for the different oil types.

An example of what gear box oil to use is Castrol MTX SAE 75W/80 API GL4, which is stated as "transmission oil designed for twostroke motorcycle transmissions, in both road and racing applications". and Referring to the SAE numbers above the data sheet also states that a SAE 75W/80 is a "gear oil equivalent to a 10W-30 engine oil viscosity rating."(5973_MTX_106228_2007_11.pdf).

The official modern specification however is 80W-90 API GL-4, specifically the Agip Formula Moto Gear 80W-90 (TDS0304_e.pdf, the oil has lately been renamed to eni Rotra Bike 80W-90).

When choosing gear box oil it is important to get oil suited to two-stroke engines and wet clutches. Contrary to the recommendation of some mechanics, under no circumstances should an oil rated API GL5 ('for severe conditions') ever be used in a Vespa 2-stroke gear box. The additives employed in API GL5 oils can be corrosive of the soft yellow metals used in Vespa 2-stroke gear boxes.

Synthetic or Mineral?

Obviously there was no such thing as synthetic oil when the Vespa 2-stroke engine (essentially little changed over the decades) was first installed in a Vespa. Hence the question arises whether synthetic gear box oil may be used in a Vespa 2-stroke manual gear box with advantage. There are two schools of thought here. The first argues that the Vespa scooter is equipped with a very primitive engine, and therefore it is foolish (and a waste of money) to use anything other than the cheapest and crudest gear oil to be found (including, as the story goes, chainsaw oil!). This school of thought even goes so far as to argue that using anything other than mineral (dino) oil will probably cause damage to the scooter engine. According to this argument, synthetic oil is too slippery for the primitive Vespa 2-stroke engine, and using it will result in either the clutch slipping or clutch damage (cork disintegration or similar).

The second school of thought argues that if a scooter is kitted, using a modern synthetic oil will not injure the clutch, but will probably help the scooter to perform better. The advantages of using synthetic oil cited by this school of thought are: smoother gear changes, reduced wear of gears, longer duty cycle of oil, greater heat resistance of synthetic oil to disintegration of lubricating properties, and greater cleansing action. Subscribers to this school point to the statements published by various manufacturers of synthetic oil to the effect that: 1) synthetic oils are not "more slippery" than conventional mineral oil (the molecules are simply more uniform)  2) hence, clutch slippage will not occur as a result of using synthetic oil 3) a synthetic oil designed for use with wet clutches will not cause damage to the clutch, since this is precisely the application for which such an oil was designed. Here is an article defending the use of synthetic oils in wet clutches: http://www.harley-performance.com/wet-clutch.html.

While there is no lack of alleged anecdotal evidence (from various scootering authorities) of engine damage caused by using synthetic oil, there is also equally abundant and compelling evidence of scooterists with kitted engines who have used synthetic oil for a long time without any clutch slippage or damage caused to their engines. The verdict on using synthetic gear box oil in 2-stroke Vespas therefore must be: caveat emptor - let the buyer beware. Every Vespa scooterist should be aware of the fact that the Vespa was not designed to be used with synthetic oil; that there is a school of thought opposed to its use in 2-stroke Vespas; and that many scooterists claim to have first-hand experience or to know of others who have suffered engine failure allegedly as a result of using synthetic oil. Knowing the risks involved, and being aware of the arguments put forth by the other school of thought, a scooterist may make a more informed decision as to whether or not he should use synthetic gear box oil.

Further reading

http://en.wikipedia.org/wiki/Gear_oil state that "It is extremely important to note that EP additives are corrosive to yellow metals such as copper or brass in bushings or synchronizers... It is important that purchasers check the oil against the vehicle manufacturer's specification to ensure it does not contain any aggressive chemicals that may attack yellow metal gear components"

http://www.castrolmoto.com/en/oil_selector.php recommends "MTX Part Synthetic" which is API GL4 (as per Vespa recommendation). 

Corrosiveness is related to temperature http://www.opieoils.co.uk/pdfs/Redline/SynthGearOils.pdf states in advertising their own product that "The extreme pressure chemistry used in many gear oils can be corrosive to brass and bronze used in synchronizers and bushings. Most gear oils are corrosive at temperatures of 200°F... A corrosive gear oil can shorten synchronizer life by half and can also contribute to rust problems."

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Paul McIntosh,
Mar 25, 2015, 1:18 AM
ą
Paul McIntosh,
Mar 25, 2015, 1:18 AM
Ċ
Paul McIntosh,
Mar 25, 2015, 1:18 AM
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