Lubricating Skate Bearings
A sealed and greased bearing is preferred to one that is oiled due to the fact that skateboarders simply do not clean and re-lube at intervals even close to necessary for oil use. Meticulous racers should run an oiled bearing, but only if they clean and re-lube throughout the day. A greased bearing is basically maintenance free, but can be cleaned and re-lubed if need be. See SKF's site for cleaning details. The bearing cartridge is not to be packed full of grease, only enough to coat the actual balls (20% of open space). To much grease causes unnecessary friction.
NTN has an excellent article on bearing lubrication HERE and another article on care and failure analysis [HERE. Torco has an informative artical on grease HERE. SKF Grease LGLT 2, polyalphaolefin (PAO) synthetic base oils grease with a lithium soap thickener (NLGI Grade 1.5) is considered the standard bearing grease for general skate use. This is some expensive stuff. Maintenance Products Direct caries it and it sells for $80 per 1.0 kg. I have a tub of this an would be willing to sell some.
If oiling bearings for max speed, exactly 2 drops of an ISO 13 oil should be applied. may be ok. Basically you need a grease rated at MIL-G-23827. To understand some more details regarding grease specifications read THIS.About the best way to get grease or oil into a skate bearing is by using a syringe. I found some cheap suppliers online. Microtools has a good selection of syringes, at good prices, with a variety of needles to choose from (15 guage works well).
