Never use additives, I use Pennzoil and have in all of my engines and the ones I built. I use 20-50 in my old Pan. The oil passages in the timing case going to the bottem end are about .050 If you use a heavy oil like a 50 weight or 60 weight, when you start her up cold, that oil is thick and takes a little time to get up to the top end, {anybody ever wonder why the front rocker, front head, gets worn out first or when you always seem to get that noise from the front jug??] This is the reason why. My best advise is to run the 20-50 oil, course I'm talking about a fresh engine. Additives like STP are good for old smokee cars and such, to keep em going a little longer. Just my opinion. cole

 

No on the additive.Just step of to 60 weight If you think 1/4 Qt is a lot,personally I don't think thats anything to be concerned about.

 

Keep running what serves you best for the climate and top it up as needed. Top end needs doing anyway but in the mean time you won't be possible causing extra wear in the bottom with too heavy an oil on cold startups. A leakdown test will give you a better idea on where the problem lies. Could be just guides and valve seals.
Ron