The clear caps were designed to allow you to see the oil level on oil filled hubs. DRV requested the white ones to use on greased bearings so people wouldn't be confused. Either one will work just fine just be aware that grease will sling into the clear cap so you still will not be able to see anything after a few miles.
The rubber cap in the center allows the same style cap to be used on EZ Lube hubs, oil filled hubs or std, greased bearing hubs.
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