OK boys and girls - Lube Oil 101 - Viscosity Modifiers (VII)
I haven't seen the composition of STP for likely over a decade, but this product was, and most "engine honeys" are, based on known petroleum technology.
This may get a little long and techincal, but I'll Coles Notes it as best I can.
To manufacture a "multigrade" oil (5W-30, 10W-30 etc), you typically start with a lighter base oil (5W or 10W) and add a Viscosity Index Improver (VII). A VII is basically a "chopped rubber" product, or a long chain polymer. These polymers basically "wrap" themselves up at cold temperatures, and so stay out of the way, but at hot temperatures "spread out" and link together to form a web, which traps oil molecules, making the oil act like a thicker oil. So far, so good. VII's also tend to improve the effectiveness of oil rings, usually cutting down on oil migration into combustion chambers.
The problem with VII's is that all are prone to shearing...heavy forces, such as gears, cams, tappets, etc. tend to cut the VII's into shorter and shorter chains, causing the beneficial properties to degrade over time. Different VII's have better or worse "spread out & link" properties, and better of worse "shearing" properties.
Many engine honey's are made from PolyIsoButylene (PIB). PIB is still commonly used as a tackifier in things such as Chain Bar Oils, or Spindle Oils and some greases. PIB is a very effective VII with a very short life.
Back in the early days you could "boost" your 10W30 into a 10W40 or 20W50 by adding things like STP. For a 60's vintage race car, this added viscosity and lower oil consumption was a great idea for a 2-4 hour race. It didn't matter that most of the effect was gone in a short period of time, as you were going to dump the oil anyway...I'll let you determine how much effect was left after 200+ hours. (not much)
These days, there are better VII's than PIB, and they're typically in your oil now. Diesel engine oils tend to have more shear resistant VII's, synthetic oils (PAO's) tend to need lower treat rates of VII's. Most engine honey's will at best provide a short term increase in High Temperature viscosity, typically at the expense of Low Temperature viscosity, at worst will cause unexpected problems, like Red01's case above.
So...is there ever a time to use them? Absolutely...extreme situations may require extreme measures. A real world example given to me by a Maintenance Super at a large mine. Driving across the Prairies at 2:00 am in his MGB, he had an oil ring break. Engine is now using oil like crazy. How do you get the oil consumption down and limp the thing 10 hours home. Well, this might be a time for STP or similar. Your going to have to rebuild the engine anyway, so go ahead and use something. But that doesn't mean that it will provide any long lasting benefit in normal conditions.
Next weeks lesson...HTHS (high temp high shear) viscosity and temporary vs permanent viscosity loss.