Some clue to the timing of GM's testing might be indicated by the revised
ZDDP levels between SE, SF and SG
1942 -- ZDDP introduced at 300ppm.
1955 -- API MS ZDDP at 800ppm.
In the 1970's there were problems with oxidation causing oil to thicken.
ZDDP is also an anti-oxidant, so ZDDP was increased to...
1971 API SD except GM, ZDDP at 1000ppm.
1971-80 -- API SE, ZDDP unchanged at 1000ppm
Used by GM 1971-on, but cam/ tappet wear protection
proved inadequate and there was a rash of warranty claims.
1980-88 API SF, ZDDP increased to 1500ppm
(compensating for an issue ?)
1989-93 API SG & 1993-96 API SH ZDDP 1200ppm.
(issue better ?)
Despite the ZDDP limits noted above, some oils of the day were available
with higher ZDDP levels, like Mobil 1 20W-50, which was 1600ppm ZDDP
(1600phos / 1700zinc). I can't explain the loopholes.
1997-01 API SJ & 2001-04 API SL ZDDP reduced 1000ppm
(back to 1971 level)
2004-now API SM
1) ZDDP 600ppm minimum level set
2) ZDDP reduced to 600-850ppm maximum for FIVE
select SAE 30 weight and lighter oils:.... (0W-20, 5W-20, 0W030, 5W-30 & 10W-30).
3) ZDDP 1200ppm maximum for all other non-selected
grades. Manufacturers are "allowed" (not mandated)
to use 1200 ZDDP (1200 ppm phos. / 1300 ppm zinc).
The problem with SM is that increasing the ZDDP levels for the non-selected
grades is at the oil company's discretion. Some do, some don't, some go a
little, some go a lot. Many that do offer a higher ZDDP option select a
token viscosity grade instead of doing so across the range. For those SM
oils, the label tells you nothing about how much ZDDP is in the oil. It
can vary from 600ppm to 1200ppm and still be an SM oil. The inconsistency
creates a mine field for owners of vintage and motorsports flat tappet
When API SM was introduced, just about all mainline US oil company brands
and grades dropped to 800 ppm without an announcement. It wasn't until the
public outcry from vintage and motorsports groups that some oil companies
began to bump up ZDDP levels in select viscosity grades. The API SM spec
didn't change, it's still SM. But the way the oil companies chose to work