Originally Posted by steitsma
Can you just add STP Oil Treatment to your oil and get the same result as opposed to using the more expensive oil with the higher levels of zinc?
The problem with the STP oil treatment is its not just ZDDP its a bunch of viscosity modifiers which make your oil much thicker IE heavier weight. The reason for this is STP markets this stuff for people that have older high mileage engines that have larger bearing clearances and burn oil which will benefit from the thicker oil.
Unless your 4.0 has a huge amount of miles on it (like over 150,000) all the STP additive is going to do is cause your engine to get less oil flow because of the thicker oil.
There are products like ZDDP plus that do add zinc and phos without any modifiers but they are around $10 a bottle IIRC.
The other issue is the quality of the oil you put the ZDDP in. There is a reason that the European car markers got away from the API certification because its standards allowed a wide variance from really good high quality synthetics all the way to really cheap crappy oil. Which basically has allowed the oils of today to not be as good as the ones 10 years ago.
So to deal with this car makers like BMW, Benz, ETC created the ACEA rating system. So if you want a quality oil to put the ZDDP in then buy a ACEA rated oil. But your back where you started because the 5W-40 TDT is a ACEA rated oil
Honestly with whats at stake just spend the extra $15 when you change your oil and get the right oil.
On a side note I've ran Mobil one 10W-30 in my 01 4Runner for the first 100K then switched to 5W-40 TDT till where I am now at 300K. I pulled the valve cover yesterday to replace the seal and the engine looks brand new, no varnish or oil staining. And this is a engine that has been plagued with the sludge issue.
Mobile one and the 5W-40 TDT oils are very much worth the money.