A producer of saleable hydrocarbons filed for a refund of sales tax paid on the purchase of equipment and related items used in the processing of the hydrocarbons. The request covered purchases made between March 1, 1997 and February 29, 2000. Since a mandatory exemption permit became effective November 1, 1998 and the taxpayer did not follow the mandated procedure of applying for the exemption permit, the taxpayer was only entitled to a refund of sales taxes for the period prior to the required exemption certificate date. (Apache Corp. v. Oklahoma Tax Commission, Oklahoma Supreme Court, No.97907, June 15, 2004)