I'm trying to increase the oil pressure in a 96 Nissan Sentra 1.6 Liter
engine. The oil light comes on with engine hot, idling in Drive, goes
out when put in neutral. I just changed the oil from 10w-30 to 20-w-50,
and the light has not come on again    :)    But it's still making a
lot of valve noise when hot, so pressure seems to still be aproblem.  I
had a mech put a gauge on it and was reading ZERO when hot and idling,
and only 15 when revved to about 1500. I know, don't drive it.

I think there is still something else that can be done to improve the
oil pressure. I have received conflicting predictions about whether the
op wouild be affected by it.

There is supposed to be an "oil pressure regulator" valve on the engine
block, which is covered by the oil filter. When changing the filter, I
noticed, on the engine block,  there is only a hole with a spring
resting loosely inside. No cap, poppet, lid, etc covering the hole.
Just a wide open bypass hole. Looks like the "op reg valve" broke off
and went somehwere, I hope into the garbage after an oil change, and
not up into the engine !!    :(   don't know.

I have orderd the part from Nissan and will get it tomorrow.

My question is whether the bypass being wide open constanly is
affecting the op.

My ASSUMPTIONS (some redundant, admitted)  are :

A.   The oil flows from the pump directly up to the filter inlet
(unless pump bypass is stuck open!)

B.   Once the oil is entering the oil filter area, most, or all of it,
is going right into the open bypass hole, and ignoring the filter,
since it is getting resistance there. Maybe 10 percent of the oil is
going thu the filter if rpms are high enough.

C.  The oil pressure is its highest immediately after it leaves the
pump, and DECREASES as it encounters bleedoff opportunities on its
journey through the engine.

D.  The oil pressure AFTER an obstruction,  like an oil filter, will be
lower than the oil press in front of the obstruction.

E.  An obstruction acts to INCREASE the oil pressure in the oil in
front of (before) the obstruction. (Same as D, I know)

F.  The idiot light op sensor is located AFTER the oil filter and also
AFTER the bypass hole.

G.  The oil flowing thru the filter and thru the bypass hole end up in
the same channel, the oil from both sources recombines somewhere, like
two streams feeding into a river, then goes on its way up to the head.

H.  Installing the "oil pressure regulator valve"  will INCREASE the op
BEFORE the oil filter, but may actually DECREASE the op after the
filter, since now the oil will be forced into the filter (obstruction)
intead of just flowing thru the wide open bypass hole as before. BTW
this is a new, clean unplugged filter).

    Thanks.  Some folks have told me installing the new bypass valve
will boost the oil pressure, even  AFTER the filter, but I don't see
how. I hope it does, but I don't see how.

Thanks for any replies !!