thanks guys, but i got my answer from XRAY Support. their online support is awesome. i always get a response very quickly. and this is so detailed and accurate that i didn't even have any extra questions afterwards. i am more and more impressed with XRAY. here's their response to my question.
"Dear Sir
Thank you for your email and your interest in Xray products.
The differential has been built per factory specifications and this way it has been build for several years by well trained factory assembly staff with great results. The diffs are being build and set-up for ultra smooth operation. All factory team drivers have been using the original diffs without modification or re-assembly which is a proof that the diffs are build ready for any high competition conditions. There is a thin and sufficient layer of grease put on the large diff shims, there is sufficient amount of grease applied on the carbide balls and there is sufficient amount of grease applied on the axial bearing as well.
You will not find any extra amount of grease in the differential because XRAY differential has been build for smooth operation and features only the neccessary, appropriate and sufficient amount of the grease. If a large amount of grease would be applied, this will make the operation of the differential not as smooth and efficient like it is now - built per factory specifications.
Moreover XRAY differentials feature the XRAY Dust-Covers which seals the differential perfectly and no or very minimum of dust can get into the differential. Therefore even a small amount of grease is sufficient to make the differential work smoothly and perfectly for a long time.
Please note that XRAY has been using a very special grease which is very thin and transparent so it may look the differential is dry. XRAY strongly suggest NOT to use any other grease than the original XRAY grease.
To summarize the answer - XRAY differentials are NOT dry. If you do not see any overflowing amount of grease in the differential do not worry, your differential has been built properly with thin and sufficient layer of thin grease for smooth operation of the differential. If you believe you need more grease than is the factory standard, then please use the included grease in your kit but remember that a larger amount of grease may make the differential run not that smooth. You can purchase the XRAY Grease under part number #30 9500 Silicone Grease
At the end, please remember if you want your differential to last long you need to break-in the differential properly: When you rebuild your diff, even if you use the same diff rings and diff balls, or start using a new diff it is best if you take the time to run it in before tightening it beyond a loose setting. A well run in diff will last many weeks racing without needing a rebuild. Moreover it will run smoother throughout that period. In fact if well broken in the XRay diffs will feel smoother than some of the one-ways on other cars...
So how do you break in a diff?
Well the trick is to run the diff very loose the first time you run it. If your first run is during a qualifier then try and aim for the loosest possible setting where the diff does not slip ON THE TRACK. The latter is in bold because a diff that is set so that you can still push through it a bit manually will usually not slip or only slip the first two laps on the track!
If you have the luxury of running in the diff during practice you may want to run the diff slightly more loose still so it slips slightly throughout the run. Don't overdrive the car.
Ideally for the second and third run you only tighten the diff a little bit compared to the previous run to allow the diff to get fully seated in. After that you can tighten it further to medium or even tight settings if necessary.
If I can help you in any other way just let us know. We will be more than happy to assist you or help you out.
XRAY MODEL RACING CARS
www.teamxray.com
[email protected]"