I haven't read the thread but know a bit about this from working on ships with hydraulics and having a cadet put multigrade in the Bow Thruster hydralics.

If the oil is synthetic you will be fine. The steering may feel different and lumpy due to the different viscositys of the separate fluids, but any specific seals will not be affected if it's a synthetic oil.

50 miles should not be a problem, although a complete new charge of hydraulic fuild will be needed.

In our case that was 6800 litres. A bargain at 85 quid for 205 litres

Is it a customer of yours Maria?