Grease is preferred on the steel bearings (and in fact is what comes in them to begin with). A bit thicker and more resistance when new or newly greased, but after a couple runs they loosen up nicely. Longer lasting lubrication.
Clutch bearings is a different story. Ideally you want to de-grease/blow out and lube with a light oil to avoid grease blowing out in your clutch and making your shoes slip, creating an ugly heated situation. If you oil, however, your steel bearings won't last as long, and will need to be re-lubed.
For CERAMIC bearings, use a light oil. They last 5x as long, because they have self lubricating properties, and light oil goes ALOT longer in ceramics than in standard steel.