I looked and I couldn’t find a use of the term “Ölchemie”, however I am pretty sure that for all other oils, there were never enough chemical processes to have a need in finding a name for a branch of the chemical industry, as for “Petrochemie”. But yes, per the Etymologie link that I looked at, “Petrochemie” comes from greek “petros” for rock, and “ol” for oil - technically (and the article mentioned the same) it should be “Petrolchemie” which makes it closer to the “Alchemy” joke the previous commenter was making :)