As BB indicated above what you are experiencing the the cylinder choke a a typical ABC, ABN engine tighter bore at top dead center. The thing about one brand of plug allowing free spinning is just confusion about how you applied the starter.

Avoid dry cranking at all cost especially on a new engine. For your first start on a new engine with considerable choke do not hand prop and use only a starter being careful to back the prop up aginst compression before applying the starter switch. Now my preference for an engine with considerable choke is to remove the plug and hold a finger over the exhaust outlet with the throttle wide open then spin the engine for a few seconds or until fuel spits out the glow plug hole. Then reinstall the plug and go for the start. heating the engine can help however I have not saw one yet that absolutely required it. Some high performance racing engine yes. Make sure your starter battery chord is not excessively long and that your battery is properly charged.

The OS #8 plug is a midrange plug I would suggest in the case of OS plugs either #3 or #6, The six is the new replacement for the three.

John