Testing Slick 50 and STP
We also tested Slick 50 and STP Engine Treatments and STP Oil Treatment,
each in three cabs. (Slick 50 costs $17.79 per container. STP Engine
Treatment has been discontinued.) All three boast that they reduce
engine friction and wear. The engine treatments are added with the oil
(we used Pennzoil 10W-30). They claim they bond to engine parts and
provide protection for 25,000 miles or more. We used each according to
instructions.
The STP Oil Treatment is supposed to be added with each oil change. It
comes in one formulation (black bottle, $4.32) for cars with up to
36,000 miles, another (blue bottle, $3.17) for cars that have more than
36,000 miles or are more than four years old. We used the first version
for the first 36,000 miles, the second for the rest of the test, again
with Pennzoil 10W-30. When we disassembled the engines and checked for
wear and deposits, we found no discernible benefits from any of these
products. The bottom line: We see little reason why anyone using one of
today's high-quality motor oils would need these engine/oil treatments.
One notable effect of STP Oil Treatment was an increase in oil
viscosity. It made our 10W-30 oil act more like a 15W-40, a grade not
often recommended. In very cold weather, that might pose a risk of
engine damage.
