99 gmc safari tic toc just like a clock when warm and idleing

SOURCE: low oil when engine warms up gmc sierra 1995

30-40 psi of oil pressure is fine for many vehicles including your GMC. I am assuming that the truck has more than 100,000 miles on it and still has the original engine in it. By rule of thumb you only need 10 psi per 1000 rpms in a typical street driven engine to give adequate oil pressure to keep the bearings from oil starvation. The oil pressure is typically higher when initial start up occurs because it is more viscous due to the temperature of the oil as the engines oil warms up the visconsity thins out and the oil flows more smoothly. The drop in oil pressure is a normal occurance. Above 45 is more than required for a daily driven street engine. I believe that the specs on that engine only list 30- 45 psi.

SOURCE: 2004 GMC Sierra High Oil Pressure

if it is electrical oil pressure sensor I would start there. The best way to see if you have a major problem is to use a mechanical oil pressure tester. Take off the oil pressure sensor and put the mechanical tester on. The tester should come with a Gage. If it comes within your trucks oil pressure range than all is fine and replace the sending unit and see if that don't help your problem.If the tester runs high then the oil pump is bad or your cam-bearings could be the problem. But with cam-bearings you would usually get a low reading.

SOURCE: 1992 gmc chevy 350 wont idle when warm

This could be a number of things but i think that your problem sounds like a vacuum leak or an EGR problem. Try listening for leaks and try un-plugging the EGR solenoid valve one time.

SOURCE: 2003 GMC Envoy - High Oil Pressure Reading

Sounds like an oil pressure sensor. Here is a diagram to locate it for removal. Best of luck.
Greg

http://www.chiltonlibrary.com/content/images/21202/images/21202_GDIA_g518.pdf

SOURCE: low oil pressure 93 GMC P/U w/350 motor w/205,000 miles gauge

While engine is running, open the cover where you add oil to engine and see if you see a good flow of oil, if good flow, could be as easy as replacing oil sending sensor. got to realize that if poor maintenance history your engine may not be building enough pressure at iddle to move your hydraulic operated valves, only when you rev it up it will pump enough to run smooth. if running engine too long, you may cause more damage to other engine components such as crankshaft and cam shaft bearings. if you can spare a few bucks, have a good mechanic check your oil pressure with a mechanical gauge. another cheap option is to replace oil pump. last resort if you love your truck as much as we do,replace engine or rebuild it. At 205,000 this is a normal symptom, other than timing chain problems, most engines don't make it that far. hope this helps!