I have a 96 civic EX with 110k on it. It has been a great car, the only
thing I have ever had to replace was the alternator and battery. I replaced
the timing belt not too long ago just to get it out of the way.

The engine however many times make a deep knocking when in low gear or going
up a hill, especially when the engine is cold. The dealer could not find
anything and suggested moving up an octane. I am curious if I should some
STP treatment to clean out the engine. I am not very car savy---cant even
change my own oil---so is the STP stuff safe?

Any other advice to take care of the engine? All my driving is stop and go
and I get the oil changed every 3k.