The tan color would signify that the o-ring material is probably not nitrile (Buna), neoprene or Viton they are usually black. The only tan rings I have seen were epychlorhydrin (probably not spelled right). My guess there is something in the TCH that could adversely effect nitrile, but Viton should work in that fluid. 'Bout the only thing Viton doesn't like is water, especially hot water and some amine based corrosion inhibitors but you usually don't see these in commercial hydraulic fluids. Also, Viton will take temps to about 300-350F.