The pilot was on a cross-country flight when he noticed the oil pressure was decreasing rapidly. He elected to make an off-airport landing, but the engine seized before he could reach the selected area. Subsequently, the aircraft was damaged during a wheels-up landing. Initial touchdown was on a gravel road and the aircraft hit a berm before slowing to a stop. An examination of the aircraft and engine revealed an oil (quick) drain valve (Aeroquip PN: AE80664H) had been installed on 7/10/87. It was installed with a 4-3/8 inch hose that extended below the engine reservoir, in the vicinity of the landing gear. Subsequently, the drain valve was pushed open by movement of the landing gear as it was being retracted. This resulted in loss of oil and subsequent engine seizure. This drain valve was not approved for installation on this model of aircraft. Piper Service Letter 910 and AD 81-11-02/R1 specified use of another valve (Piper PN: 492-172V).