Is it bad to do a engine flush on a older motor with about 120,000 miles on it

if you use seafoam or anything that'll break down the sludge inside the engine, dont leave that stuff in there too long or it'll mess up some of your oil seals. also, some of the grime in the engine, combined with old cracked seals, kinda help prevent oil leaks from happening i heard, since the grime fills in the cracks in the seals.

Yeaaa I wasn't to sure on doing it but my buddy decided to try to do something nice for me and put this Lucas injection cleaner inside my engine. Iunno what he was thinking but I thought that was the best for the engine, I left the motor flush inside for like 8 mins. Then drained it and did a regalar synthetic oil change. Seems to be running fine, just driving slow