Indeed we have seen a lot of info on the reduction of ZDDP in motor oils online for the last couple of years.
The bulk of which unfortunately are myths and parrotted misinformation accompanied by anecdotal evidence, if any at all.
I always do my own research and thinking regardless and what follows are my personal conclusions to date.
To figure out what amount of zinc/phosphorus (ZDDP) is safe for my 235 and 261, I first was curious about the history of concentration levels.
During the 40s phosphorus levels were around .03%
In the mid 50s, when more aggressive cam grinds and higher valve spring pressures became more common, phosphorus levels were increased to around .08%
Test showed that although scuffing is reduced during initial break-in, too much zinc/phosphorus is not a good thing for the long term.
At 0.14% wear is increased over the long term and at 0.20% the ZDDP starts to attack the iron in the cam.
By the 70s heavy duty oils had about 0.12-0.16% - Cams had very high lift and spring pressures were extreme.
Levels have been reduced in the last 10-15 years to protect exhaust catalysts.
Today typical gasoline motor oil has a maximum of .08%
Same as oil during the 1950s...
Diesel oils, like Rotella used to have about 0.11% phosphorus, which has been reduced in 2007 to the current .09% in 15W-40 multi-grade.
Straight-grade like SAE30 remains unchanged and is still in production.
The people who promote/push the ZDDPlus additive advise against the use of diesel oils in gasoline engines, to no surprise.
After mixing their additive with current standard motor oil as recommended, the resulting level of phosphorus is 0.19% !
That is about twice the concentration compared to what has worked fine for 50 years...
I'm rather skeptical that their product will help my engines.
There are hours of reading info on all this online from various sources.
The historical data I provided is derived from the GM Powertrain Fuels and Lubricants Group.
The levels on Rotella oils are derived from independent oil analysis data.
The ZDDPlus info is available on their site.
On my 30+ year old 235, I use Rotella SAE30 all year round.
My fresh 261 gets the same oil in summer and Rotella 15W-40 during the colder months, with around 30 degrees as the coldest cranking temperatures.
I believe there is plenty of ZDDP in current Rotella oil.
The extra detergent and dispersant additives may not be ideal for high performance engines, but work well for our motors, especially when older with more blow-by.
To be sure, I will have my oils analyzed before the next oil changes and will update this post if needed.
