Yes, SAE 80W/90 or SAE 90 EP works. However, it's important to note that the BCS's transmission requires the non-sulfur type for yellow metal/bronze gears because there's bronze in the BCS's transmission. If the tiller attachment's transmission contains bronze and requires non-sulfur gear oil, I have not heard about it. EarthToolsKY's video on BCS transmission oil change states this information, and their website links to a chart of gear oils tested to be okay to use with yellow metal/bronze gears.
What lubricant do you use? Is it the same as in the tractor transmission?
Yes, SAE 80W/90 or SAE 90 EP works. However, it's important to note that the BCS's transmission requires the non-sulfur type for yellow metal/bronze gears because there's bronze in the BCS's transmission. If the tiller attachment's transmission contains bronze and requires non-sulfur gear oil, I have not heard about it.
EarthToolsKY's video on BCS transmission oil change states this information, and their website links to a chart of gear oils tested to be okay to use with yellow metal/bronze gears.