Synchronizing Gauge (Manometers)
Mercury tools are inexpensive and never need adjustment, but mercury is poisonous and environmentally incorrect. If you're not careful the mercury can get sucked into the engine. It won't hurt the engine, and the instructions that come with the tool should explain how to purge it.
Dial gauges don't contain mercury, but good ones are expensive.
Various home made Synch Gauge instructions abound on the internet. Here's a really nice one courtesy of Jon Pardue:
Jon Pardue's Synctool (PDF)