If you drive at freeway speeds the cap can be blown around and around and will come off. Toyota has a TSB about this and there has been recall for aerodynamic oil fill cap failures, it's T100-21 dated on Jan 11, 2002. The caps have a stronger spring, now, and the problem has been fixed. Mechanics have been cautioned to check the cap and install the updated version, if needed. In some cases you will get an error code because of the vacuum leak created by running the engine without the cap, ER-12.11. This is referenced in the Toyota Master Mechanics refresher course outline, dated March 3, 2005. These cautions do not apply to the Sequoia after March 1, 2005 or any V8 4Runner, as they came equipped with the updated oil fill cap from the factory. Remember, as stated in the TSB, turn the cap until you hear it click once, they continue to turn it until you have it in the correct alignment. If you do your own oil changes, you can stop at your nearest Toyota dealer and have them confirm that you have the updated cap and that you know how to properly align it. Also besure you insert the dipstick correctly and have the raised high/low markers pointed away from the engine block. Pleas refer to TSB 100-03.