You can put seafoam in ANY ENGINE, even 2 cycle! I've been using seafoam for very long time (since it was $4 per can!)
In my car, I do 1/3 in gas, 1/3 in oil, and I suck 1/3 into my intake via a vacuum hose. I let the engine run for about 60 - 90 seconds before it stalls (yes it will stall out) usually, you need a friend to help by keeping the engine revved so it doesn't stall while sucking the seafoam into the intake. If it stalls early, it's not a big deal.
After the engine stalls, wait about 10 minutes then start it back up. It will be a jerk and try to not stay running, but you just have to show it who's boss (and I don't mean Tony danza)
When the engine starts running, be prepared to see clouds of smoke coming from, well everywhere! Mostly the exhaust. Then, you might want to drive around until the smoking stops.
That smoke is produced by the carbon build up being burned up that was "broke free" by the seafoam.
Seafoam isn't really an additive, it's a cleansing agent. That's why it causes engines to run so much better, it douches the carbon buildups from the engine.
In my lightning, I plan on putting equal parts (probably 1.5oz into my gas, my oil, and into my velocity stack (I'll pour it in SLOWLY) with the engine running. When my tank of gas is gone I'll change the oil.
You can leave it in the oil but it defeats the purpose because the point is to remove build up, not to break it free and let it hang out in the oil.