97 Accord oil light

Have a 97 4cyl Accord. Believe it or not..bought it at a YARD SALE! That was 100,000 miles ago. They day after we bought it, the oil light started blinking...so I got a guage 2c what was going on. When I first start it in the morning, pressure is about 80 pounds. After it reaches operating temp, it drops to 50 pounds at highway speeds, but drops to zero or maybe 2 pounds at stop lights. There are times the oil light doesn't come on until about 30 minutes to 45 or so...of driving...then it starts flashing. It's been doing this now for 100k miles. I change the oil AND filter every 3500 miles. What causes this oil pressure fluctuation?

You'll always have higher oil pressure when the motor is cold, the oil is heavier cold and thus has higher pressure.

If the oil light flickers at idle once warmed up that's usually a sign of main bearings that are sloppy enough that they bleed off oil pressure faster than the pump can deliver at low rpm. These engines usually rattle pretty good when started cold though, as the mains don't retain much oil film while sitting. Another possibility is a worn oil pump, or you are using oil of a grade too light for the application. If you're using 5w-30 in a motor with 150,000 miles or more, that's too light.

They used to say a very broad rule of thumb was 10 psi per 10 mph, but I've found that's rarely the case. Anyway I think Honda says 10 psi at idle is normal. Lotta gauges can't really display readings lower than that reliably. So accounting for some wear, if you show anything at idle, it's probably ok, long as it doesn't get noisy and rattle at idle.

I'd change to the next grade heavier than what you currently use and see what you get...

i had a similar problem in my 1997 honda accord and my oil screen was clogged... look into it.