I grew up on flat head engines.   I learned my trade at the hands of
some older guys who were older  than I am now.   I have never heard
STP as anything other than 'stop that ping'.    For all you or I or
anyone knows the original German product was Schrasad Toudant
Platza-ersatzen which translates as 'gluen downen dos damnitz noisen
pistonzee'.  Most German product names don't seem to translate well
into English and they get spin doctors to dress up a name with an
entirely new set of words.    Like a Sonata GLC - where GLC translats
into English as 'good looking car'.   Or if your a computer nut find
out what S.A.P. realy means.

I was shown how to identify piston offset when the piston head wasn't
marked as 'front'  with a notch or an F.    I learned that some
engine's pistons didn't have pin offset that among  others included
Studes.   As a mechanic for all those years, I've never worked on a
Stude V engine.   Not even the last model years that came out with a
chev small block.

I learned that most every oil additive on the market at the time was
mouse milk, including STP.   And that STP was more of a glue than an
oil thickener.   I knew people who used the stuff to quieten those
pesky knocky noises from sloppy rod bearings (babbit),  Sloppy worn
pistons from a 'rering job',  or 2 cans in the oil burner you wanted
to sell.   I saw one guy put it into his cast iron powerglide to
soften the hard shift.   The trans didn't shift at all below 0.   I've
used the stuff to lubricate a valve guide knurler and have mixed it
with lubriplate to glue needle bearings into place on assemblies.  I
was only dumb enuff to use in my engine once.   If you have low oil
pressure, find and repair the problem or buy thicker oil.  If you want
better lubrication,  buy better oils.

I went with my father and uncle to a dealer to look at a new Stude.
The sales guy was throwing in STP with the sale.  This tells you there
was an issue with quality control or there was a need for the product
on a new engine.  That was in the middle 50s.  As an aside, my uncle
cursed that car for not starting when hot.

I do know that all the rebuilders I ever did business with hated the
stuff because it never got cleaned out of a casting and the stuff felt
like crap on the hands.    The guys who think this is a good product
are the same ones who rant on about Fords being soooo much better than
Chevy without actually knowing why or voting a particular party into
power because 'that's the way we've always done it'.   Before you
shoot off about how good something is, find out what exactly is in
the product what actually happens in the short and long term.

If it ever warms up I'll go into the garage and look up my old data
books and see if there is any pertinent info on the old flat engines.
If you have a flat engine from the late 40s to early 50s, measure a
piston to see if you get any pin boss offset.