Spark Plugs

[CarsByCarl]

Originally Posted by Greg Curtis

Thanks Carl, didn't know all the oil companies had dropped zinc in their oil. I thought Kendall GT-1 still had zinc in it. What about STP Carl?

Don't really know about GT-1, but it's definitely worth checking before you use it.

Here's some info on STP I researched about a little over a year ago before the diesel oil specs were changed. For the record, I don't use STP. I use VP Racing BI-30 oil for flat tappet break-in. After break-in, there are a number of companies making oils with high ZDDP....Valvoline, Brad Penn, and VP being a few.

STP has 1959ppm of zinc and 1814ppm of phosporous. So, that would be diluted to 391ppm zinc and 362ppm phosphorous when added to five quarts of oil.

Rotella 15-40 which seems to be accepted as having enough zinc to prevent flat tappet wear contains roughly 1200ppm zinc and 1100ppm of phosphorous.

Current passenger vehicle oils on the market now are SM spec and contain approx 870ppm of zinc and 800ppm of phos, therefore adding a bottle of STP should bring that level up to roughly 1261ppm zinc, and 1162 phos.

The conclusion I've arrived at here is that running with a bottle of STP added to SM spec oil is as effective as running 15-40 diesel oil in our effort to maintain the service life of flat tappet cam engines.

Interestingly, I've also read that adding STP to 10-30 will increase viscosity to about 15-40. So, adding STP to 5-20 should bring it up to 10-30 if you are looking to maintain that viscosity.