Could be a big problem if left. The clutch uses mineral oil not DOT fluid. It needs cleaning out straight away otherwise it'll damage all the seals and hoses from the lever to the slave.
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Really?
Yes, hydraulic systems running mineral oil use different materials for hose linings and seals, using DOT will damage them and cause premature failure if left.
I used to race and still look after a few Race XC Mountainbikes, Shimano use mineral oil in their top end brakes, had a few where people used the wrong fluid. Obviously you'd need to leave the stuff in there a while, but ultimately it will damage the system.
