I add the STP Highly Concentrated engine flush to the old oil right before the oil change.
I don't drive the car at all during this flushing and just idle the engine for 15 minutes to let the flush cleanse the inside of the engine, and then I drain out the old oil.
Also, to test the flush, I had an oil change done with new oil first, then added the flush, ran for 15 minutes and drained.
The brand new oil + the flush came out very dark so I know it's doing some significant cleaning.
From my observations the oil comes out much darker when using the STP Super Concentrated High Mileage Engine Flush (15 fl. oz.)
then when I use the Liqui Moly Proline engine flush, so I think the STP is a stronger cleaner. STP says on the bottle that their engine flush is safe to use with every oil change, so I'm hoping that is true. STP is a US company founded in the 1950's, so I do trust the company when they say it's safe to use with every oil change.
I'm doing 3 STP engine flushes in each of my vehicles, so after that the engines should be really clean inside.
After the 3 STP engine flushes in each vehicle, a perpetual short OCI of 4k/6 months with a good low priced full Synthetic 5W-30
that has a high dose of detergent additives (SuperTech with Detergent Calcium additive = 1,382ppm) should be all I'll need to keep the engines clean for the life of the vehicles.
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