Originally Posted by Espada
Originally Posted by bluesubie
IMO, the affect that an oil treatment would have on the cold cranking ability of an oil is not known because it is not something that is tested. We sometimes see comments on uoa's along the lines of "the oil sheared to a 5W-20", although what the lab means is that the oil sheared to a 20 grade because they likely did not actually perform a cold cranking viscosity test.
In those temps, I'd probably be a little more comfortable running a 10W-50, if you have access to it, and adding STP to that would
probably have less of an impact on cold cranking than adding it to a 20W-50. Otherwise, the only thing that I would recommend is to just try it and see what happens. And regardless of viscosity used just take it easy until the car is at operating temp. Do you have a temp gauge?
edit: Which STP oil treatment are you asking about? I just looked up STP Engine Stop Leak.
Viscosity: 3,150 cSt at 40°C
296.5 cSt at 100°C
Adding 20% of that to an 18 cSt oil puts you
above a 60 grade oil at 100C according to a viscosity calculator.
I have a temp gauge and i am talking about this stp
I remember as a kid seeing ads for STP back in the late 60's that stated STP helped engine internals by coating parts with treated oil that wouldn't run off when the vehicle was not in use. They claimed the results were reduced wear on cold starts, and an extra margin of protection. All I know is adding to a 50 grade oil would produce a pretty thick finished lube, and cause the starter and battery to work harder in cold weather turning the engine over.