What Oil Should I Use? On the subject of oil, we recommend that engines with roller bearing big ends are run on monograde oils i.e. single viscosity sae 40 in winter and sae 50 in summer. This is because this type of oil can withstand the high loads experienced in a roller bearing big end where all the force is through just one or two rollers. Monograde oils should not be confused with ‘straight oil’, which means oil without additives. The principal additive in modern oils is a detergent that is used to keep deposits in suspension until they reach the oil filter. They will also loosen existing deposits and as there is no effective filter this can be disastrous in an engine that is full of ‘gunge’ accumulated over the years.
Engines with shell bearing big ends can use either a monograde or a modern multi-grade oil. The multi-grade oil will lubricate the shells more quickly when the engine is first started which is when most of the wear takes place and so is more suitable for short journeys; it is also cheaper and more easily obtainable. Monograde oil will leak less; the engine will run cooler and quieter, is available as a straight oil and is more original!
Do not forget to clean out the oil tank when rebuilding the engine.