Oil temperature chart


This is an interesting chart. It shows “how hot is too hot?” for both your engine oil and your tow vehicle’s transmission. It also shows how a pattern of short trips can mess up oil by not letting it get hot enough.

Transmission fluid for tow vehicle. For most modern cars, tranny and diff fluid are marketed as “lifetime fill”. Some years ago I bought a Ford F-150 with 112k miles on it. Previous owner was a petite girl that had put on lots of freeway miles. Truck looked brand new. Even the driver’s seat looked new, because she’d weighed so little.

I had an open trailer back then, so not much to pull. I was playing around with an Android App called Torque that would let you set up gauges fed by the OBD2 data stream. That app showed me that my tranny temps were surprisingly high. But I didn’t know what was normal for tranny temps. All I knew was that the tranny was supposed to be “lifetime fill”.

I had a shop replace the tranny fluid. Temp went from 235deg down to 190deg. Dang, what a huge difference. Imagine how much additional friction was going on inside of the transmission to generate such additional heat. According to the chart, the fluid change likely saved the transmission.


The opposite is also bad, too cold can also mean that oil pressures stay too high and engages the bypass, preventing flow to the bearing. I am not worried about that in my newly SoCal weather, but back in MTL, with temperatures reaching and passing -30C, you could feel how horrible the startup was for the engine. Worst it was if you were doing short distances.

One aspect also to consider is moisture control, don’t remember exactly my thermal dynamics, you need bring the temperature of the oil over waters the boiling point or close to get rid of the moisture that the engine attracted overnight. Again, short trips to the store in cold temps don’t this well.

Finally, Oil cooler+Thermostat is a must, I believe our cars come with it. But like the chart shows, overcooled oil as bad as overcooked oil.


I thought the bypass only bypassed the filter (and I assume cooler if you have one) but still sent oil through the engine? At least that was the way it was on SBCs and BBCs.


I might be wrong, but the engine has 3 bypass/regulators.

Inside the filter(depending of manufacturer)
On the pump.
And within the galley


The filter bypass doesn’t have anything to do with oil flow to the bearings. If the filter is too full or the media has collapsed and plugged, the filter’s bypass just allows the oil to keep flowing past the plug.

The pump’s bypass is to protect the pump’s drive shaft. If the car is in a cold climate one shouldn’t be using oil so thick that the pump has trouble moving it on start up. IIRC, it opens around 80psi.

The bypass that hangs down from the block is called the “Oil Pressure Relief Valve” (OPRV), I think, and connects to the block’s oil galley, the channel that feeds the crank main bearings. It sets the max pressure of oil in the system to ~70-75psi.

There’s one more valve. The oil filter adapter assembly has a thermostat in it. Oil doesn’t flow thru the oil cooler until it reaches 180(?)deg.


I wouldn’t say the filter bypass has no effect on the overall flow. It certainly not used often, but I wouldn’t be surprised if it pops open at startup for a fraction of a second.

Also, are you sure the pump regulator opens at 80psi? That would assume a max differential of pressure of 5psi between OPRV and Pump. I would assume at least 5psi at low pressure and would increase with pressure. Bentley recommends at least 7psi at idle.


Now my question is: Is viscosity linear? Can we correlate temperature, pressure and viscosity? Or is it logarithmic or exponential?

Because to me it looks like 20w-50 is twice as thick as 5w30 at 100C, which should translate to 2=1 pressure gain if it follows PV=NRT… lol


I don’t recall the opening pressure of the valve on the pump, but I’m pretty sure it’s not less than 80 nor more than 100psi. Chuck Baader took a look at my oil pump when I had problems with the OPRV. He described the valve on the pump as a clumsy affair not designed to modulate pressure, like the OPRV can, but to be more of an open/closed off switch sort of thing that was only to protect the pump. He opened up the valve and smoothed out it’s cylinder so it would operate a bit better.

Temp and visc isn’t linear. Oil gets thick really fast as the the temp drops. But once oil gets hot, there isn’t much visc change with increasing temp. That is to say big change 30deg to 0deg, hardly any change 200deg-230deg.

I didn’t understand your point re. max differential pressure of 5psi at low P and more at high P. These things aren’t precision instruments.


If you assume the engine oil passages is one tiny but very long tube. There will be a pressure differential from the start to the end. Now if you assume your starting pressure is 85psi. I would image your end pressure to be a lot less then 80psi. That is why I have a hard time believing the pump’s relief valve is set close to 80psi.


The only oil pressure relief I have is the valve on the pump. I have no OPRV. The problem, I’d argue, with what you’re describing is that we’re talking about a valve that is at the pump while the place we all measure it is a bit farther away, at the OEM oil pressure measuring port, which is darn close to the OPRV.

So sure, the valve at the pump is prob a little higher than 80psi, but if the location where we universally sense pressure sees 80psi because of the pressure drop due to the flow drag in the 12" of oil channel in between, we’re kinda into a distinction w/o value. But I agree, that strictly speaking the pressure at the pump would be slightly higher than the pressure sensed at the sensor port and OPRV. And I agree that the pressure drop would be a function of visc and flow rate.