Public-TV patron Herbert Schmertz, vice president public affairs of the Mobil Corp., announced this week that he is leaving the oil company May 1 to form his own public relations and production firm. Schmertz "plans to devote a substantial amount of his time acting in an advisory capacity on behalf of Sen. Robert Dole's presidential candidacy," according to the announcement. During Schmertz's 22 years with Mobil he spearheaded the company's "Masterpiece Theatre" and "Mystery!" series. Producing station WGBH-TV in Boston says it has a funding commitment from Mobil for the two series through calendar year 1990 and is "currently negotiating an extension of that commitment through 1993."