The wrong viscosity of oil in your engine.
Low oil pressure or fluctuating oil pressure.
The oil pressure gauge is going to fluctuate on a Toyota Tacoma or any automobile. The oil pressure is less when the auto is idling and more when the auto is going down the road.
symptoms of high engine oil pressure
No oil pressure, engine noise, engine failure.
Engine noise.
High discharge pressure and low liquid line pressure
Try O'Reilly Auto.
under the bonnet
As the engine speed increase so does the speed of the oil pump. the faster the pump turns the higher the pressure becomes. When changing gear the engine speed drops along with the oil pressure.
I know in my '97 Gmc Jimmy the oil sending unit was bad when I had the same problem holding oil pressure. When I'd start my truck it had almost 40 lbs of pressure that would slowly fall off to nil unless you accelerated. Then the pressure would rise somewhat.
Low oil pressure Low cylinder pressure Anti freeze coming out of the tailpipe (sickly sweet smell) Water in oil Oil in water Frothing or bubbling in radiator
The oil pump turns more at higher revs, this is then producing more oil pressure, so when you change gear the revs are in a lower range so the oil pressure drops.