Synthetic oil is definitely not neccessary for the TR1, and you often hear stories that it made the clutch slip. So use normal mineralic 15W40. Also be sure, that the clutch cable and the mechanism under the little aluminum cover on the left side of the engine are well oiled and free moving, and that there is enough clearance within the cable and the pressure rod. Eventually use stronger springs, for example from Lucas.

Bye,
Georg