Oil is all you need. But I would never start a Square Four after a rebuild without putting about a pint of oil in the sump. Yes, it will smoke a bit and throw some oil out but if you have an oiling problem such as an oil lock or the pipes connected the wrong way round it will safeguard the engine. And assuming all is well, the smoke and oil will be cleared very quickly so you can look forward to many happy miles on the bike.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.