I use one from
http://www.oilamatic.com/ on my Cessna 180. So far I've been using it for over 20 years and probably 650 hours.
The installation was not difficult. The package included everything.
It's STC'd for that make and model and others.
I use it on start-up and shut-down. On shut-down, without it, oil pressure drops off before the prop is fully stopped. The prop then stops abruptly. But with the preoiler, using it, the prop goes to the next compression stroke and bounces back. Proof that it's still got lubrication.
For start-up, I turn it on when I begin my pre-start checks, and by the time I'm ready to start, there's 50 psi. Because of the electric drain, I shut the preoiler off and immediately start the engine. I don't know how necessary that is, but it's what I do.
Another use is after oil changes, to pressure-test the system.
Works great and is highly recommended.
The only reason I'm not planning on installing on on my RV-3B is the very tight engine bay. Otherwise I'd do it for sure.
The vendor told me that it's not certified for use while the engine is running but that it doesn't have any ill effects. It was merely the additional certification cost that kept him from certifying that.
Dave