More correctly I should have stated that besides the Olympus OM's, the Pentax LX uses OTF on all exposures - manual, aperture priority autoexposure and TTL flash.

Tim, The manual for the OM4T specifies an autoexposure of about 4 minutes - which is what I have managed to get on mine. So for the several minutes exposure you have gotten is that also autoexposure or just bulb?