Use 'so long' in informal situations when parting ways, like with friends.

Casual Farewell

'So long' implies you believe you'll see the person again in the future.

Expectation of Return

'So long' carries a warm, friendly feeling when leaving.

Point out a noticeable gap since the last encounter or action taken.

Often conveys a feeling of missing someone or something from the past.

'So long as' sets terms for an agreement or permission.

Using 'so long as' doesn’t guarantee something will happen, but it's a requirement.