You said "crankcase" so I am assuming you mean a 4-stroke. But, back in that day there were lots of "250r" 2-storkes around, so I will talk about both. (If you have an 89 Honda 250R 2-stroke that is in good shape, you are golden! If so, see you in the Honda forum!)......
If this is a 2-STROKE, what you are lubing is the TRANSMISSION! (This is NOT the "crankcase". The crankcase gets lubed by the premix you mix in with the gas!) I like one of the transmission specific lubes, like Maxima MTL, Motul Trans Oil, or Honda's HP Trans Oil. You can also however, run a 4-stroke motorcycle engine oil that is API SG/JASO MA.
If a 4-STROKE, run a motorcycle specific oil that is API SG/JASO MA! For an old machine, I would probably run Maixma Premium. (Just so you get even more confused, there are now some 4-strokes (Honda TRX450R, and Honda 450/250/150 dirt bikes...God Bless Honda for this one!!!) that seperate the engine and transmission oils, and in that sense these transmissions have identical lubrication requirements to a 2-stroke trans. The engine side of these machines, still would use a 4-stroke motorcycle oil, preferably a high quality ester synthetic.)
Some essential reading...and why API SG/JASO MA is important:
ThumperFaq: 4-Stroke Oil
Oils Well That Ends Well, Part 1 - Sport Rider Magazine
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