| If the level has dropped the fluid must have gone somewhere, so yes a new slave cylinder seal is probably required. ProTwins do an X-ring seal that works much better for about £2. Check the condition of the slave cylinder bore very carefully. If it is badly scored it may need replacing in which case an aftermarket such as Evolutione would be a good idea, which most people will recommend you do anyway. If the original cylinder has only light scoring you may be able to clean it up with fine wet and dry.
And yes 5.2 is good preferably a synthetic based fluid. | 
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