So my indie reckons I'd needed a new rack last week cause the fluid was black. It turned heavy and I was a sad man. They thankfully changed the fluids and re oiled it and it worked a treat. It was back like new for a week. Then today it's started feeling heavy again. I'm worried taking it back to them will have them insist to do a £1020 plus vat rack replacement.

Any ideas on hat this could be?
Which fluid should I buy:

Cold Climate Power Steering Oil?