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The fitting that "typically" threads into the slave is a pipe thread (where it threads into the slave). In that case, new or old fitting, use a teflon paste. Don't use Teflon tape as it tends to shread and enter the fluid stream.

There are "straight thread" fittings that use a copper crush ring or some type of o-ring but as far as clutch slaves they tend to be the exception, not the rule.