Just remembered I have not updated my thread and in case someone else have the same problem they can see what works best. Before I'll start tearing the engine apart I have decided to install oil pressure gauge instead of changing sending unit. I followed instructions from here:
http://www.4runners.org/writeups/gaugeoil/index.html
The only thing I would add that I have installed 4A fast acting fuse as recommended by Autometer. Now I have exact numbers to go by. At cold start I get 80 psi, at idle once it's warmed up it's 15 psi.
I also found interesting thread:
http://www.yotatech.com/f2/oil-light-flicker-131175/
The official Toyota 1998 4runner Owners Manual, Chapter 1-5 p.69:
"(d) Low Oil Pressure Warning Light
This light warns that the engine oil pressure is too low. If it flickers or stays on while you are driving, pull off the road to a safe place and stop the engine immediately. Call a Toyota dealer or qualified repair shop for assistance.
The light may occasionally flicker when the engine is idling or it may come on briefly after a hard stop. There is no cause for concern if it then goes out when the engine is accelerated slightly.
The light may come on when the oil level is extremely low. It is not designed to indicate low oil level, and the oil level must be checked using the level dipstick."