'86 560sl gas tank capacity

[FreedomArtist]

The fuel gauge on my ’86 560sl has never worked…so in order to keep track of fuel usage, I set my trip meter to Zer0 and when it reaches 200 I fill up the tank. My questions are the following:

1) How many gallons does a 560sl tank hold? (I’ve never driven until empty).
2) What octane is recommended for 560sl (I don’t have a manuel)
3) Where can I purchase a manuel?

Thanks a million for your response.