Wow, that's weird. My 99 has black from the factory. I know mine is 5/16. If your's is grey it's probably printed on there either 6 or 8 mm. Which is 1/4 or 5/16. I buy the OEM filters off ebay when they pop up. But a clear plastic unit from the autoparts store would work. They're like palm size, a little smaller in diameter than a soda can.