1998 Chevy Van Oil Pressure Gauge Fluctuation

Last week the service engine soon light came on. I got the van in for an oil change and to check why the light was on. Before getting it into the shop the oil pressure gauge started fluctuating, this was a couple days after the light came on. When I start up, the oil pressure is fight in between 40 and 80. As the engine starts warming up as I brake and slow down the gauge goes down. When I'm idling at intersections it is in between 0 and 40. When I go again it is either at 40 or goes up higher. Now when it was at the shop they found that the catalyst effiiciency was low, but nothing else showed up. The vehicle is running fine, but the oil pressure gauge is still fluctuating. Why is it fluctuating, is it connected to the low catalyst efficiency reading?