Absense of lead *may* cause exhasut valves to wear a bit faster than they would with lead. Not much, but some. Since it's a mechanical lifter (I presume) you are checking clearances anyway, so no problem. So you wind up doing a vavle job at 40K instead of 60K. BFD. New valves are better alloy than old ones, so after a valve job you're good to go.
Skip the lead, enjoy the ride!
George
