Is Seafoam the reason why my car is shifting funny?

About 6 months ago a not-so-close friend of mine (we get along its just we dont hang out unless hes working on my car) did an oil change for me on my 02 ford focus and suggested Seafoam for my car. This was the second time he suggested using it, the first time was a hit when we mixed some of it with my gasoline. My engine ran a little smother it felt like. This time he said you can mix it in with the oil and i figured why not since it worked so well last time. A couple days after and ever since my car has been getting worse with the way it shifts gears. My car seems as if it lost power because when i step on the gas it doesnt get up to speed as quickly and sometimes i have to press down on it with a little muscle and then it will slam into 3rd gear. It also will look as if its going into fourth gear when i give it some gas when its really going into second. Any clues to what is going on?

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