A B737-400 Captain reported a 'dirty socks' type of smell in the cockpit after extending the landing gear. Odor traced to a hydraulic leak in a main landing gear wheel well.

Narrative

A hydraulic odor; or 'dirty sock' smell was detected in the flight deck and passenger cabin shortly after landing gear was extended. Flight landed uneventfully and taxied to the gate where maintenance was notified. Both Flight Deck Crew members and one Cabin Crew member smelled the odor. It caused my eyes to burn accompanied by a bitter; metallic taste in my mouth. According to the Maintenance Technician a hydraulic leak was detected by him in the main landing gear well. Apparently the hydraulic fumes somehow made it into the flight deck and cabin after the landing gear was extended. After landing; air conditioning packs were turned off and flight deck windows were opened for ventilation after slowing to a safe taxi speed. I considered donning my O2 mask but due to our proximity to the ground (less than 1;500 AGL) I didn't want to be distracted from the primary task of safely flying the jet. Maintenance needs to insure that future events like this do not occur. This is my eighth hydraulic exposure at work that I am aware of since November of 2006.