I have an '08 Silverado 1500 4x4 with 50,500 miles. Yesterday I jacked it up in the garage and changed the fluid in both the front and rear diffs. I ...

I have an '08 Silverado 1500 4x4 with 50,500 miles. Yesterday I jacked it up in the garage and changed the fluid in both the front and rear diffs. I used a suction device to pull the fluid from the rear fill hole and the front had a drain plug.

Both diffs had really dark, almost black fluid in them. Is there a factory addative used that would do this to the gear oil? I know it has never been changed since new. When I wiped some spillage up with a rag it looked almost like there was silver anti-sieze in it.

BTW I filled the front with 80w-90 GL5 Valvoline and the rear with 75w-90 Synth. Valvoline.

Everything sounds normal, with those miles the diff lube breaks down and turns dark, and the anti-sieze looking materal is normal gear wear. We change our gears in our late model dirt stock car dozens of times during the season. You should see the sieze in that lube. You should be just fine,,,

50k seems a bit too long. Do you tow a lot? I would aim for 25k diff changes.

I'm relieved to know it is normal for it to turn dark. Compared to the nice, clean golden fluid going in it was nasty. I thought maybe it was either contaminated or there was a break-in additive put in by the factory. On the plus side, there were not many metal shavings on the magnetized drain plug.

I intend to change the fluids regularly, but the truck is new to me so I wanted to do all the fluids now just so I know when they were last changed as I don't have any records from the previous owner.

The tranny is next. Although with the snow in the forecast it may be a couple more weeks until I get to it. Nothing worse than rolling around on a cold, wet garage floor while trying to avoid getting petroleum products in your eyes.

Definitely true. Winter time maintenance is tough. I don't blame ya for waiting. I hate those cold wet floors!