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I just completed the 100 service on my (Bad Boy) zero turn. It has Hydro-Gear ZT-3200 units. The drain oil on the right unit was a golden color as I expected. The left oil was BLACK. In the bright sunlight, It appeared to be a grayish-black. I was shocked to say the least.
I emailed Hydro-Gear about it but they're not responding.
What could cause the oil to turn so black while the right unit looked normal?

I will venture to say you are making more left turns than right, hence left transmission is working harder & creating more heat than right transmission.

Could be the transaxle is overheating because a partially only disengaged brake caliper or debris on covering the transaxle. Blacken is usually a sign a severe overheating condition where the oil is burnt.

OK. I'll check brake adjustments. Thanks.

Two things
1) left side gets more debris on the case so needs to be cleaned more often
2) left side is on the lack side of the belt so slips more then the right
lest side usually go first .