I had my BMW M52 engine completely rebuilt. The rebuilder retired and I’m not sure what’s the break-in procedure or what oil I should be using?
When running a "new" or rebuilt engine for the first time after I had built it from scratch I would use it gently for 200 miles - not exceeding 2500 rpm approx or putting it under heavy load. Then over the next 500 miles I would increase the rpm occasionally to 3500 or 4000. After 1000 miles Then it was fine.
As for oil, I would use the manufacturer's specification and often better. On a re-built engine I would do the first change at 500 miles (oil & filter) and then follow normal change intervals.
If the engine has not been started before, I would remove the spark plugs and spin the engine over on the starter until the oil pressure had built up and there was evidence that it had come through the rockers or valve train (sometimes can be seen by removing the filler cap).