[MrInBetween]

The drop-down box from Search on the menu bar does AND searches, and the Advanced Search does OR.

That is, the ordinary Search is the same as putting searchstring1+searchstring2 into the Advanced Search, which returns posts (or threads with such posts, if you select the Threads radiobutton) that contain both words.

[EDIT]
Even if you start with a search from the drop-down menu, you always go to the Advanced Search if no matches are found, and a hidden consequence of this is that your first search uses implicit AND and the second one implicit OR. Try entering a string of two words in the drop-down search box, the first one a word that you know exists on the forum, the second one a nonsense word that doesn't; you'll get no results - but if you then leave the string unchanged and press 'Search Now' at the bottom of the page, you get all the results that contain the first word.