Back on the ex Australian Military Scania 4-series, 2002, P140c, 4x4, (50k kms) and the front right planetary gear hub oil came out dark and with a bit of a silver tint to it. There is a whir coming from the front but not 100% it is coming from the right side. I have been told that the 4x4 all have a whir. True?. All other planetaries and gearbox oils came are pristine but changed them to Penrite full synth 80w140 "Pro Gear" Oil. Rechecked the front right planetary after another 2000kms and it was once again dark and a bit shiny. (no metal flecks etc just very small dusty size). Planetary gears look good from external inspection. Thoughts? Change the wheel bearing? cheers
No need to worry. Everything sounds normal.
Whirring gears are not usually a sign of trouble, unless the noises change significantly over a few thousand kms. In fact, experienced mechanical engineers consider "singing gears are happy gears" (see p.15 of this technical review). Years ago, I found my transmission whirs a bit after changing the fluid several times in short intervals, probably because the gears are clean and smooth; a friend's transmission now whirs slightly after a complete rebuild.
The dark color is not a concern. The right-front wheel may just take more of a beating than the other wheels. As long as there is metal dust in the used fluid, and not flakes, the wear is normal.
Full synthetic oil is never a bad idea. If you'd like to greatly enhance the oil, add Archoil AR9100 at the recommended dosage of 3.2 oz. per quart. The benefits of nano hexagonal boron nitrate have been studied intensely over the past 15 years and have proved invaluable to many large commercial operators of heavy trucks and machinery. I've been using it in the crankcase of several cars (1.2 oz./qt.), which has in all cases improved compression and eliminated a similar problem of darkened oil.
One last point. We are talking about differential gears, not planetary gears, which are located solely in automatic transmissions.