ZDDP additive for an old slant six

Recommended Posts

On some other Dodge forums, people suggested using a ZDDP additive to the engine oil.  Is this a good idea?  Ed

Share this post

Link to post
Share on other sites

Waste of money. Just use a diesel oil, API CI-4 or CJ-4. These have plenty of zinc. If you want to minimise wear, use a synthetic oil such as 5W-30 and forget about zinc. Any synthetic oil is better at minimising wear than ALL mineral oils.


Read Richard Widman's paper on oil at


but here is the paper to save you the trouble of looking it up.


Share this post

Link to post
Share on other sites

Spinneyhill is absolutely correct.  The new diesel engine oil specification is now API CK-4, which is usually paired with the passenger car motor oil (PCMO) specification of API SN.  A dual-rated heavy duty engine oil would therefore carry the API CK-4/SN (or API CJ-4/SM, etc) designation.  The concern with API SN is that it requires 30-grade oils to also have a maximum phosphorus (the antiwear component of ZDDP) level of 800 ppm.  That is why now it is now more difficult to find a 30-grade HDEO with an API CK-4/SN rating.


I've been using Petro Canada Duron SHP 10W-30 in the 225 in my 65 Barracuda.  Duron SHP is approved for API CK-4/SN for the 15W-40 grade while the 10W-30 grade is approved for API CK-4 and SUITABLE FOR API SN because it uses the 15W-40's additive package but cannot be approved for API CK-4 because of its 1200 ppm phosphorus content.  I have not been able to get other HDEOs (like Shell Rotella) to confirm that their 10W-30 grades use the same additive package as their 15W-40 grades.  See Heavy Duty Engine Oils.


My 1965 owners manual recommends the 20W-40 grade for ambient temperatures are consistently above 32°F (see Chrysler Oil Recommendations).  I've upgraded to a 75 psi hemi oil relief valve spring and I've found that my original oil pump has no problem generating 75 psi above 2000 RPM in hot weather with 10W-30.  Using a thicker oil (like 15W-40) just means that more oil blows off back to the sump without reaching the bearings.


If you're looking for a good engine oil for your slant six, there is no need to use anything heavier than a 30-grade oil (ie, 0W-30, 5W-30. 10W-30).  If it has low oil consumption, a synthetic would be a good choice (eg Mobil Delvac Elite 222 0W-30).  Excessively thick oils tend to run hotter and could cause unnecessary engine wear in colder weather.  I believe HDEOs generally have higher HTHS ratings than PCMOs and would therefore offer better bearing protection.


See Engine Wear.

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now