If the seals have degraded a bit they can taint the fluid colour, although you get compatible mineral oils in green that work fine.
Seller got back to me and said it sounded like his work. He's a hydraulic engineer in naval industry and reassured me that regardless of what some I ternst experts might say, mineral oils are all cross compatible.
@snottyotter started
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If the seals have degraded a bit they can taint the fluid colour, although you get compatible mineral oils in green that work fine.