Mech's seem to get a real rubbery one about Seafoam.
Is it just fuel additive, like something similar to Lucas Oil?
Do I just pour it into my gas tank as a gas treatment?
What will it do/help with?
Seafoam is a fuel additive which has several uses. Its primary purpose is to clean the fuel system of varnish and fuel deposits. It also cleans carbon from your combustion chamber to help reduce emissions and allow your vehicle to run better. You can also run it in your engine oil to remove sludge deposits. The following describes how you utilize the product in each of the different ways:
Fuel Additive: - Dump a can of Seafoam into your gas tank just before you fill up (yes, at the gas station). When you fill up the tank, it will thoroughly mix in with the gas and work as it should. In this process, your injectors are the main part you are cleaning.
Oil Treatment: - Use about 1/3 of a can dumped into the oil filler tube about 100 miles (or less) before an oil change. You need to run it for a little while so it can do its thing of cleaning the internals of the engine, but not so long as to cause issues.
Intake Tract Cleaning: - This part is a little more involved. For a complete rundown of how to use it this way, please read this forum post. Generally though, this process is as follows (I wrote this in another answer here):
First, this is a two person job. Second, pull the vacuum hose from the brake booster (not from the engine side). If you have a small funnel, put it into the end of the vacuum hose. Have someone start the car. They will probably need to rev the engine during the next part, but only enough to keep it running. With the engine running, slowly dump about 1/2 of the can into the funnel and let it get sucked up by the engine. This will cause volumes of white smoke, but not to worry. This is normal. When you get down to about 1/2 to 1/3 of the can left, dump the rest into the funnel and as soon as it is sucked in, have your helper shut the car off. Let the engine sit for about 10-15 minutes. After the wait is over, start the car back up. This may be a little difficult and again will cause a bunch of smoke. Keep the engine running until the smoke disappears. You have now successfully completed a Seafoam treatment. This will clean all of the carbon out of the intake tract, from the valves, and from the combustion chamber.
NOTE: This answer in no way is an endorsement of the product mentioned, it's just describing the product and its uses.