Drain bit out until the stick and the electronic system are reading "okay" level.

A quick search for SLK320 engine info found:
http://www.slk32.com/pages/engines(usa)/engines2.html
but it doesn't list any capacities.

This page http://www.slk32.com/pages/oilchange/oilchange.html says:
"The 3.2 liter engine uses 8.5 US quarts. It is recommended that only 8 quarts be added initially. Run the engine and check the level. The oil canister/filter can hold about a quart of oil so the level will drop after the oil is circulated for the first time. Add oil as needed."