Oils to Use in a Classic Bike




Engine Oils

Once running-in is complete, use whatever engine oil you feel comfortable with. If you believe that only “authentic” non-detergent mono-grade oils should be used in old engines then that is fine but buy good quality oils, renew it frequently (every 1,000 miles or so) and remember to clean the sludge from the bottom of the sump periodically and clean out the sludge trap once in a while.

If, like me, you believe that several decades of development in oil technology should not be ignored then feel free to take the benefits of that. Personally, I use high-quality, fully-synthetic multi-grade (20w50) oil with a full flow, renewable fine oil filter. I change the oil and filter every 3,000 miles or so. The benefits of this approach are that the oil provides better protection, it does not deposit sludge, either in the sump or in the oil passages and the oil circulates faster at start-up. The oil is more expensive but I change it much less frequently because the filter keeps it clean and because high-quality synthetic oil breaks down much more slowly than mineral oils.

I have noticed that the oil pressure at low engine speeds tends to be lower when multi-grade oils are used but the flow rate tends to be higher, which has cooling benefits.

However, synthetic oils must not be used during running-in. They are so effective that they prevent the necessary bedding-in of the components and will greatly extend the running-in process.

Gearbox Oils

The Bedside Book recommends the use of engine oil in the gearbox. That's fine but, if you use modern engine oil, it is a mistake to use that in the gearbox. That is because modern engine oils use additives that are OK in the engine but may be harmful to the bronze parts in the gearbox, particularly the expensive kick-start quadrant. Personally, I prefer to use a modern light gearbox oil (SAE90) but it is very important that it does not contain EP additives. The right specification to look for in a mineral gearbox oil is GL-1. I use a modern 75w90 fully synthetic gearbox oil from Red Line but other brands are available. If you choose that route, you must read the small print carefully to ensure that the oil is for use in a manual gearbox and does not contain any additives (e.g. phosphorus) that might be damaging to bronze. Synthetic gearbox oil is expensive but can be changed much less frequently than mineral oil.

Rear Drive Oils

A heavy (SAE140) gear oil is essential for the longevity of the rear drive on a Sunbeam. To avoid corrosive damage to the very expensive bronze crown wheel, you need to look for the GL-1 specification. Personally, I use a modern 90w140 fully synthetic oil from Red Line but other brands are available. If you take that path, you must read the small print carefully to ensure that the oil is suitable for use with worm gears and does not contain any additives (e.g. phosphorus) that might be damaging to bronze components. Synthetic gear oil is expensive but can be changed much less frequently than mineral oil. The biggest advantage of using synthetic oil in a sunbeam rear drive, apart from reduced wear, is the dramatic reduction in power-sapping viscous drag. The difference becomes immediately obvious when you wheel the bike along in neutral.

A Note on Synthetic Oil

Synthetic oils are quite effective in finding weaknesses in your engine/gearbox's oil seals. If you change to synthetic oil, you need to make sure that you have a healthy set of seals. Otherwise, the oil will find its way out of every joint and seal. It is, however, perfectly possible to make a Sunbeam engine almost completely oil tight, even when using synthetic oils.