Good morning all,
I debated where to put this message. I put it here over the small-engine forum as the crux of my question is the history of ZDDP as an oil additive.
One of my hobbies is the collection, restoration, and operation of vintage engines. Most of these are small air-cooled engines, most often made by Briggs and Stratton. In one of the old-engine forums, a gentleman was looking for information on oil for a Briggs engine where the manual called out for "nothing heavier than SAE 20."
His question: was the SAE 20 in the mid-50's the same as SAE 20 today?
I answered back noting that viscosity measuring technology had not changed, but that oil formulation had changed. Using my spreadsheet of oil viscosities collected from various product data sheets, I noted that the viscosity of SAE 20W-20 oils (I couldn't find data on straight SAE 20) when hot were comparable to modern 5W-20 oils.
I also noted that before he run out and buy 5W-20 for this engine, he should consider that these old engines are flat-tappet engines, and that a modern PMCO oil may not have enough ZDDP in it. (I also noted that for an engine that isn't run under load, say at engine shows, it might not matter.)
I received a reply back noting that:
These engines were designed and in production before the use of ZDDP in engine oil. So they are fine to use on modern oil. Even many flat tappet auto engines are just fine, depends on the design. ZDDP is really only needed in some of the poorly designed auto engines from the 60's through 80's.
My questions are thus:
1) When did ZDDP start being used as an oil additive?
2) Anyone care to comment on if or not an old flat-tappet engine, such as a 1935 to 1955 or so Briggs and Stratton, will do okay on PMCO?