Service manual recommends 24,000 miles or 24 months whichever comes first for normal use ('68 service manual, TH400). You replace, don't clean, the filter.
It's a messy job at best. Most important thing to remember is do not overtorque the pan bolts, if you bend the flange it will leak.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
