Gearbox oil can be a contentious topic between Lambrettas owners as everyone has a favorite. Original service manuals suggest SAE 90 gearbox oil and if you can find it, that's great. Here are a few key things to consider when choosing a gearbox oil.
Hypoid vs non-hypoid
Hypoid gear oil is designed for automotive differentials and is not suited for a Lambretta. Basically it is corrosive to yellow metals (ie bushings)so that is all you really need to know since Lambrettas use those. Get non-hypoid gear oil.
Multi-grade vs straight weight
Multi grade oils were not available when Lambrettas were originally produced. An example of a multi-grade oil is: 85w140. The basics of a multi-grade oil are that it acts like the first number (85W) in colder conditions like when you first start it and the second number (140w) during normal (warmed up) conditions. These are completely safe for Lambretta gearbox use although with the target weight being 90w, 140w is going to be way to thick. Finding a multi-grade that actually should be used in a Lambretta is not easy and not something you need to look for.
80W, 85W, 90W?
Honestly it does not matter. If you want to use 80W, feel free. If you have access to 90W then use it. All of these will work fine in your Lambretta gearbox.
SAE 30W Non-detergent
It is often said that straight weight 30 non-detergent engine oil (commonly used in Vespas) is the same as 90W gearbox oil. This is both correct and not-correct. Can you use SAE 30 Non-detergent in your Lambretta? Yes and it will work fine. Many people actually use this as a gearbox flush after they experience a contamination issue from metal shavings polluting the gearbox oil. Many old school builders will run this as the first fill on a fresh rebuild since it's cheap and they will be dumping it after 300 miles. It is generally not recommended that this be a long term use oil as you may experience increased wear over time.
Automotive oil vs Motorcycle oil
This is actually a lot more important than it might seem. What you need is a non-hypoid gearbox oil designed for use with wet clutches. Cars and trucks do not use wet clutches and generally require hypoid gear oil so it can be difficult to find the correct oil at your local auto parts store. Any decent motorcycle shop should have something that will work well for a Lambretta. If you do not have one of those, then even a Harley shop will have what you need. The nature of the Harley Sportster engine shares a lot in common with a Lambretta as they both use a primary chain and a wet clutch so Harley even makes an oil that will work great in your Lambretta.
Edited by theageofindustry