I am often repairing the gearbox in Peugeot car shop, there are rubber sealed bearings in gearbox in gear box house (not in clutch side gearbox house, in other house where the rubber sealed bearings are in between 4. and 5. gear). There are never found grease in rubber sealed bearings, also the grease are replaced with gearoil. So clear the rubber sealed bearing are not 100% sealed. The reason the sealed bearing are used in gear box are to protect the steel particles are entered into the bearing.

Since Greg (gkamysz) showed differnce grade of rubber sealing are made by NSK ball bearing manufacture, the problem with the SKF bearings when i am buying the sealed , there are often marked RS and nothing other grade of rubber sealing in SKF bearings since the seller do not knew all about the bearings..
Do not rely the rubber sealed bearing are complete tight against leakage, they are made to protect the dust are entered into the bearings. The metal sealed bearings are not protected against dust such as saw dust from table saw..
At school we maked own table saw, there are rubber sealed bearings mounted against saw dust from wood and our teacher told to us the former pupil in school had jammed bearings in his table saw due saw dust from wood, it had metal sealed bearings and soon replaced with rubber sealed bearings.