Just to clarify/embellish a little if it's even needed.  The "anti-return" valve,
usually just inside of the perimeter oil inlets of the filter, keeps the oil from
draining out of the filter when the oil pump is not operating.  It probably helps to
keep the siphon effect from slowly draining oil from the downstream side of the
filter, (the filter media may not allow this siphoning to occur...don't know),
including the lifters.  This shortens the time or avoids all together the brief
period of "oil starvation" that will otherwise happen until the filter can be
refilled and the downstream side can be replenished or "pumped-up" with oil after
you start the engine.

Dana

> clacking lifter syndrome. Typically it is the result of a lifter losing
> its prime and the oil running out  of it.  ... Ways to avoid the problem is to go to
> a 20W50 wt oil, and install a Mahle or Mann oil filter - available from
> some of our List Vendors like Ken Wilford at Vanagain.Com or Ron Salmon
> at The Bus Depot.. These two filters are constructed so that the
> internal relief valve spring is sufficiently strong that oil pressure is
> retained in the oil galleries after the engine is turned off.
>
> John Rodgers