The oil in the engine serves various functions, it lubricates the
moving parts, cools & transfers heat and cleans the engine. There are
basically three types of oil available: Monograde, Multigrade and
Synthetic, the former being the oldest. The first two are Mineral based
oils with varying characteristics. A measurement of the oils ability to
flow is called viscosity, and this is shown on the can as a figure. This
is measured at normal working temperatures, so a monograde will be a lot
thicker when it is cold. To get around this thinner oils could be used in
very cold climates or during winter periods. A modern Multigrade oil
has additives that stop the oil thinning as quickly as a straight oil as it gets hot, so it gives better protection at all temperatures.
Technically, a modern Multigrade
oil at 0 degrees
Celsius the oil viscosity rating is compared to an equivalent straight
oil's viscosity and similarly, at 100 degrees Celsius, it is again
compared to a straight oil's viscosity at 100 C.
During the normal working life of the oil it gets contaminated with small particles of carbon coming from the leakage of exhaust gases through the piston rings, the carbon being a natural wastage of fossil fuels after combustion. This is why the oil turns a black colour as it gets tired. This carbon is abrasive and needs to be removed from the oil and there are two ways of doing this: a) Sludge Trap b) Filter.
a) The sludge trap is a centrifuge which consists of a hole drilled into the crank web of the flywheel. As the oil passes by the trap, the heavy carbon deposit is thrown into the trap by centrifugal force generated by the spinning crankshaft. This forms a sludge in the trap which needs to be cleaned out periodically. To clean the trap you will need to strip the engine down completely.
b) The filter consists of a cartridge which can be replaced periodically through which the returning oil from the engine passes on it's way to the reservoir. The carbon deposits are trapped in the filter leaving the oil safe to do it's job.
Which Oil should I use, Monograde or Multigrade?
If you have the older type of British engine with the sludge trap and no filter, then the Monograde is correct. This does not have additives to encourage the carbon deposits to move and will need changing on a frequent basis.
If you have fitted a filter you should us a Multigrade as this has additives called dispersants and detergents to help the oil carry the deposits to the filter and stop it from settling in the engine. These should result in a cleaner engine when you strip it down and the oil does not degrade as quickly, so extending the period between oil changes.
Which is best?
All things get better with development, and oil is no exception. In the early sixties oils came of age and the Multigrade replaced the Monograde in technical design. These have continued to develop over the years. If you want to protect your investment, fit a filter and use a better Multigrade oil. It may not be original equipment, but it will protect the engine far more efficiently.
You can either; buy the cheap stuff and change it, say every 1000miles
or buy the dearer ones and leave it in longer.
However, the problem with old motorcycle engines is that they have very little or no filtering so leaving in a Multigrade oil for a longish period means it will get contaminated with carbon and metal fragments and thus create wear, especially to plain bushes and bearings, even costly synthetic oils would not prevent this abrasive damage. It would seem the best compromise is to use lowish price supermarket Multigrade oil and change it very often. This removes all contamination and the oil remains in grade over the short usage time. Modern car engines have much better filters and then the most important factor then becomes how long the oil can be kept in before it degrades, then quality does count.
The biggest problem with straight oil is the fact that they require the rider to keep the revs and load down for 10-20 minutes before using an engine hard especially revving it. This is to allow the oil to thin to enable the correct flow rate round the engine. The flow is very restricted when the oil is cold and thick and can cause very high oil pressures and can cause pump damage even if it has an oil pressure relief valve. Multigrade oils help with that problem. So a warm-up before you start your journey with a Straight Oil is beneficial.
Modern Synthetic Oils have additives to make them cling to the bearing surfaces and do not need the same warming up process before you move off. They are expensive and should only be used with a filter. You would not normally use Synthetic Oils with old classic bikes.
Link to Car Bibles Engine Oil