If you put in 3 words, it will return posts with any of those words, but not all of them. A post can have 1 of the 3 (or however many words you put in).
So if you put in used oil analysis it would return all post used, oil or analysis.
If you want to require word, for instance, you want to search for a post which contains all three words, put the + symbol before each required word:
+used +oil +analysis
This will return posts which contain all three.
You can also use wildcards. If you wanted to search for day, daylight and daytime: day*
You can also go to advanced search to limit which forums to search, search order, range, etc. Click the search box and scroll down to the Advanced Search button.