<VV> Multi-grade vs. 'straight weight

[vairjer at comcast.net]

Charles, et al... it is true that the manuals specified multi-weight oils during the 60's, BUT> 

The actual fact of the matter is not so much urban legend as that GM did not go "public" with the fact that testing of certain brands revealed that some multi-weight oils had serious problems with "shearing" of the long chain polymers that made the often used 7W base oil into the 10W30 would result in a basically 7W oil after as little as 1000 miles. Internal memos and some service bulletins issued in the middle of the 60's recommended NOT using certain brands of multi-grade oils in any GM engine whether air cooled or not....GM did not want to go public and end up in litigation with certain oil companies, but worked privately to provide their information and guidance to those oil companies. Eventually, the problems with the polymers used were solved and "shear stable" multi-grade oils were available across almost all brands of oils. In the mean time, enough "leakage" of information reached the general public to make many folks aware of the potential of oil breakdown but limited knowledge about which brands to avoid. 


As a "Reliability" engineer at Delco Remy during the 1965-1967 time frame I read a few reports from the tech center describing the problems encountered and the recommendations for oil use in "company cars" in our fleet. 


So, straight weight oils were not as weather friendly, but they were "Stable"....eventually all mult-grade oils became far superior once the process to assure stable polymers was well known. 




Jerry McKenzie 
PCCA 
Springfield, IL