Plenty of ordinary mineral oil of the same viscosity, either straight or multigrade, as you are going to use for the initial running in.
PLUS, I use Oregon Red Fluid Grease on the bearing surfaces in the cylinder head and throughout the motor. Camshaft running directly in the aluminium casting! It stays in place if you have delays during the rebuild before the initial startup and I haven't had any problems during the initial 20 - 25 mile run, before dumping the oil and re-filling with another fill of mineral oil. After the first 500 miles you can put a semi-synthetic oil in, but I don't believe that fully-synthetic oil is necessary with these engines.
Also add Chainsaw 2stroke oil to the fuel at 50:1 ratio for the first tank full of fuel. It doesn't appear to effect the rings/valves bedding in and gives me peace of mind that the engine top end has lubrication.
Good health, Bill