Toyota Repair: 89 toyota van LE, low oil pressure, honey oil

Question
I bought a Toyota van, LE, mid-engine, 89 (pre-previa).  When the oil warms up, the oil pressure drops, and at about five miles I start loosing power.  I have not driven it much this way, and parked it when it started to act up.  It has the gauge, and it drops to below the middle section, into the lower section (top third of the bottom third), and occasionally, at an idle, the idiot light will flicker yellow.  I have added a "honey-oil," additive, and I don't see the yellow idiot light any more, but the pressure is still low.  Is this the oil pump, that is a bear to replace, or is there anything else it could be.  I believe that the top end was re-done about three years ago, when they did a head gasket, according to the reciepts.  Also, should I replace the oil pump, or just get another engine, at this point?

Thanks,
Lewis

Answer
There could be a lot of things causing the low oil pressure but the most common cause is that the oil pump pickup screen may be clogged or partially clogged, I would check this first by removing the oil pan and checking the pickup screen and either clean it or replace it if necessary, another cause that is more serious is that the engine is just tired and all the clearances in the moving parts are worn excessively and so there is a loss of oil pressure, this may explain why putting the additive in helped somewhat.