Schaeffers would be one of them, I'd imagine.
Valvoline is another really old name, but it's been through different hands....same with many other brands...Pennzoil...QS....
I know Brad-Penn claims the Brantford, PA refinery is the oldest operating refinery in NA, I think. One home to Kendall, long ago...
I assume you mean motor oil in general and not refined products?
Since sellers of motor oil are as much branders/mixers as opposed to manufacturers you are talking about the oldest brand name and said brand name may have been acquired and passed many corporate hands over the years.
According to Wikipedia, "Schaeffer Manufacturing Company was founded in 1839 ...[and] began manufacturing oils and greases to support the westward expansion during the 1849 California Gold Rush."
Also according to Wikipedia, "Valvoline was founded by Dr. John Ellis who formulated a petroleum-based lubricant in 1866 and trademarked the Valvoline name in 1873 in Binghamton, NY. Valvoline has been serving American motorists longer than any other motor oil marketer."
This makes it sound like Schaeffer came to the automobile motor oil business after Valvoline.
Standard Oil was incorporated before Valvoline, 1870 vs 1873.
The rights to the Standard name in the US are currently owned by ExxonMobil, Chevron and BP.
ExxonMobil has full rights to use the Standard name internationally.
However, I think Schaeffers is the oldest:
Schaeffer Manufacturing Company was founded in 1839 by a German immigrant, Nicholas Schaeffer, in St. Louis, Missouri, under the name Nicholas Schaeffer and Company. Originally producing soaps and candles, the company began manufacturing oils and greases to support the westward expansion during the 1849 California Gold Rush.
Oldest (probably not in business anymore):
This might be the oldest (started with veggie oil and perfume):
Company Web Site: http://www.nayaden.co.jp/energy/
Translated (check the company profile): http://translate.google.com/translate?hl=en&sl=ja&u=http://www.nayaden.co.jp/energy/&ei=3zMeT8iQMpTXiQLsk6HlCw&sa=X&oi=translate&ct=result&resnum=4&ved=0CE8Q7gEwAw&prev=/search%3Fq%3Dnayaden%2Boil%26hl%3Den%26client%3Dfirefox-a%26hs%3DiRE%26rls%3Dorg.mozilla:en-US:official%26prmd%3Dimvns