I have a 1991 geo metro it asks for sg or sg/ cd oil . what does that mean?
You may have one of the 3 Geo Metro vehicles left on the road. It has probably had all kinds of oil in it over these last 27 years. Just put the proper weight in and drive on.
@richardsoto Do a Google search and you will find articles by the oil companies that will explain just what the designations mean.
SG is an obsolete oil classification and has been superseded by SH, SJ, and now the current oil classification is SM and SN. It mostly means the type of anti sludge and anti oxidation additives that are specified in modern engines, along with additives that don’t contaminate catalytic converters. In general, SG or anything above like SJ or SM is what you need for your car. Not anything below like SF or SE.
S stands for spark ignition, such as gasoline engines
C stands for compression ignition, diesel engines.
So SG/CD meets gasoline engine standard G and diesel engine standard D.
You probably can’t buy SG rated oil anymore, but any oil that meets specification SH or higher still meets specification SG.
The one situation where a higher letter is not ok is JASO MA and JASO MB oils for motorcycles. JASO MA is for motorcycles with wet clutches where the engine, transmission, and clutch all use the same oil. Do not use JASO MB in such an engine, or any automotive oil rated SJ or higher. The friction modifiers added to SJ and higher wreak havoc in wet clutches.
JASO MB is for scooters with belt CVT transmissions or engines that use an automotive dry clutch and with separate lube for the transmission. It has the friction modifiers that are poison to wet clutches.
If it has JASO TA through TF on it, it’s a two cycle oil designed not to carbon up the exhaust and make excessive smoke.
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Good info above. Those are just what spec the oil tests to, according to agreed-upon industry testing standards. My old VW Rabbit spec’d an oil rated either SF or CC, and I always used CC b/c I thought it was a more robust oil. Never had any oil related problems. At some point CC became difficult to find, so I switched to SF or SG or whatever rating it was they were selling at the time. Oil specs are backwards compatible, so an SG or later is ok to use in a car spec’d for SF. Next time you see some oil for sale, look carefully at the labeling; you should see the rating spec printed on the outside of the bottle somewhere.