My sister'* Buick LeSabre (thinking 2K5) had an oil leak in the area of the oil pressure sending unit. The other day the pressure dropped and would go as low as 20 psi driving down the road. My guess was the pressure relief valve. We replaced the oil pressure relief valve and the pressure sending unit as well and it repaired the problem. Your problem may only be a leaking sending unit as they can go bad but thought I would also mention this problem as well.