The thickness / viscosity of the oil is measured in an SAE rating. The lower the number, the easier it flows at low temperature. For extreme cold conditions or very short journeys, a thin light oil of SAE10 performs well. For hot running conditions, a fairly heavy oil of SAE40 or 50 is suitable. Modern oils are usually multigrade. An oil with an SAE rating of 10-40 flow like an SAE10, at very low temperature and retains its viscosity at extremely high temperatures, acting as an SAE40. Multigrade oils contain additives and detergent to help gather dirt particles until they get to the filter. These are not suitable for some classic and vintage machines and their owners use straight oils (Single grade SAE). These oils cannot give the high standard of protection and performance of a modern multigrade in varying temperatures.
Always us an oil equal or above the classification of oil recommended for your motorcycle. The Japanese Automotive Standards Organization, JASO, use JASO-MA for 4-stroke petrol motorcycle engines and JASO-FC for 2-stroke petrol engines (classification system designed to address oil-requirement issues not addressed by the leading classification agency the American Petroleum Institute - API). API classification for motorcycle oils is, for example, SE for 70'' air-cooled fours and SL for a 2005 water-cooled Multi, thus SG's better than SF, which is better than SE etc.
Oil is usually checked with the bike stood upright and the dip stick resting on the top thread. Overfilling can cause damage and seal failure. Water-cooled engines usually use little, or zero oil, between changes. Air cooled motors have larger tolerances and it's not unusual to use oil when very hot. Engines with low oil get very hot.
Drain oil from a hot engine as it flows better and holds the dirt particles better. The quantity of oil in an engine is often marked on the crankcase near the filler. It's usual to renew the filter on alternate changes.
While costly, synthetic oils are tougher and longer lasting than mineral oils and are of particular value when the vehicle is in use in arduous condition and/or if the maintenance schedule is erratic.
Most cars have a dry clutch and many automotive oils contain additives that are not suitable for motorcycle wet clutches.
Operate a total loss system where the oil is burnt in the combustion chamber. Because the oil is not heated, a low ash SAE30 oil is used (except in marine application).
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