Hi,
My rear tires are loaded and I cut the grass recently with a RFM and had no problem with browning tire tracks on the grass. It has been VERY dry here, by the way. The loader and the backhoe were not on the machine at that time.
Then I used the kubota B2910 with the loader and backhoe on to carry some cut tree away from behind the house, and I later noticed brown tire tracks...but narrow ones from the front wheels. I don't think I was using four wheel drive at the time...almost certainly not...but he marks are there...
So for what its worth...my rears are R4 running 12 psi. Have to check the fronts which are also R4s...seems like the weight of the loader plus wood made a big difference on the grass.
I don't see any marks from the rears though, even with the added weight of the backhoe on the machine...
Hope this adds something to the discussion!
Bill in Pgh, PA...almost...