Some time ago I did some reading about synthetic gear oils. To my understanding synthetic gear oils are better than mineral oils at high and low temperatures - but slightly inferior at mild temperatures. The gearbox on my scooter always run at mild temperatures, which is one of the reasons I decided to stick to mineral oil at the gearbox. Since I bought the scooter I've had mineral gear oil in stock anyway, so I decided to use it on the scooter as well.
When I changed my scooter's gear (mineral) oil recently, I also found a very slight grey color in the oil. The oil color was almost as original - but a very slight grey color was visible just after the drain plug was removed.
I might be wrong - but I think the grey color is caused by Aluminum dust which came from the gearbox housing. Metal dust coming from the gears would be darker in color, and a milky oil color would be an indication of moist condensation in the oil.
So I think some moving components might be rubbing slightly against the gearbox Housing.
I am not too worried about it - the drained oil at my gearbox was still almost perfectly clean. Maybe you must just change the gearbox oil more regularly, or try mineral gearbox oil. I use 80 W 90 mineral oil at the gearbox.
My scooter has just over 16200 kms on the clock.