Best way to get a rear diff oil sample?

[wtd]

I want to get a sample of the 75W-90 Mobil 1 gear oil I run in the rear diff of my 98 chevy truck to see if I can run it longer. I don't want to change it out yet. I assume I will have to use some type of pump to remove the sample from the fill hole since this diff doesn't have a drain hole, but how far down into the diff should I take the sample from? Is there a better method?

Thanks for any tips.

Wayne

 

[MolaKule]

I take a 60-80 cc syringe and put a poly hose (8") on it as a snout.

Open up the oil fill hole and suck it out with this little pump.

You may need two draws to fill your sample bottle.

Sample when differential fluid is hot.