For single-grade oils, the kinematic viscosity is measured at a reference temperature of 100 °C (212 °F) in units of mm²/s or the equivalent older non-SI units, centistokes (abbreviated cSt). Based on the range of viscosity the oil falls in at that temperature, the oil is graded as SAE number 0, 5, 10, 20, 30, 40, 50, or 60. The higher the viscosity, the higher the SAE grade number is. These numbers are often referred to as the 'weight of a motor oil'. The reference temperature is meant to approximate the operating temperature to which motor oil is exposed in an engine.

http://en.wikipedia.org/wiki/Motor_oil

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Note added at 2 days3 hrs (2009-06-12 11:33:01 GMT)
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Natural oil wordt meestal gebruikt als antoniem van synthetic oil.

Natural lubricating oils include animal oils, vegetable oils (e.g., castor oil and lard oil), petroleum oils, mineral oils, and oils derived from coal or shale. The preferred natural lubricating oil is mineral oil.

http://www.faqs.org/patents/app/20090005277

Plantaardige en minerale smeerolie zijn te beperkt, natuurlijke smeerolie geeft maar één Google-treffer. Wellicht is "niet-synthetische smeerolie" de veiligste oplossing.