We're talking about two different things here. People hear 'lock' and immediately suggest graphite lube, but the OP isn't having a problem with the key cylinder at all. He clearly stated that he's having a hard time with the physical latching portion of the lock "once the key is (already) turned 180 degrees."

If we were talking about the lock cylinder, I'd agree that WD40 and T-9 are a terrible idea (sorry for those who have hosed your home locks with the stuff already, your clock is ticking) Silicone lubricants can work well, and are even sold by high profile lock manufacturers in their own blends (see Assa Abloy lasspray [lockspray]). In very special cases where the mechanism requires it, manufacturers provide their own (possibly oil based?) lubes. Example of this would be the Medeco lock spray. This is a special case, and I wouldn't recommend using it for any standard pin tumbler lock.

To clarify for those confused, it sounds like we're dealing with a "latch" problem. In this case, some grease would be absolutely appropriate, and necessary. If the parts are hard to reach, you could try some white lithium grease in a spray. If you have good access, then some heavier duty grease might be better, and last longer before needing service again. But do be careful to keep it away from the actual key cylinder. Right now your key is turning reliably, and you don't want to mess with that.

-Jeremy