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April 02, 2020 3 min read

The age old question, “How hot is ok?” comes up many times in conversations, tech calls, forums, and groups. Being that factory ratings are not always the best for longevity or real world use there are many things that have to be taken into consideration when deciding fluid operating ranges for an engine or transmission. Here are some of the main things: 

  • Fluid type
  • System
  • Heat cycle duration
  • Environment 
  • Condition of the mechanical components


The 6.7 Powerstroke, as an example, has primary and secondary cooling systems which operate at completely different temperature ranges. Primary stock is around 195°F whereas the secondary system is around 100°F. Therefore, saying coolant temp should be around 200°F in this application does not work so well. 

Although coolant temps are regulated by the thermostat and operate accordingly, knowing your base temperatures are key to keeping them in check. That means knowing your thermostat temperatures are key. Also, it’s recommended to maintenance and check your coolant with a hydrometer every few months to make sure the freeze point ratio is correct. Remember, water cools better than coolant.

For lower temperatures, you can add theSPE low temp, high flow thermostat. For more information regarding it and how it worksCLICK HERE


Step one is to know your oil. This is going to make all the difference so you know when your oil breaks down and when it’s additives and lubricity are no longer functioning. Mineral based oils, for example Rotella T4 and similar CJ4 rated oils, will carry a lower maximum temperature for operation in comparison to a full synthetic oil. Mineral based max temps should be kept under 250°F

At 250°F the oil begins to degrade very quickly causing many issues. Most CK4 17+ synthetic grade oils will withstand much higher temps, some manufacturers actually claim close to 300°F. These are very basic temperature figures based on general lab testing. 

Looking at this from a real world perspective there are a few things to also consider:

  • This matters because of local heat differences. For example, if your oil temp sensor is reading 220°F and it is located in or near the pan, the oil in the turbo center section will be 15 to 20 degrees hotter due to the localized heat from exhaust temperature being so close to it. 

  • Making sure that during operation oil temps reach around 178°Fto burn off condensation that builds up over time. If too much moisture is in the oil that can also affect its ability to do its job.

  • If proper maintenance isn't done and oil life is degraded its ability to withstand temperatures will be much worse. So, if you see high temperatures frequently tightening up your maintenance schedule would be smart. You should also consider running synthetic.

    Since coolant and oil temperatures go hand and hand, you can see benefits to oil longevity from theSPE low temp, high flow thermostat


    Very similar to engine oil most of the above applies with only some differences. ATF is much thinner and similar to the consistency of hydraulic fluid. In the 6.7 Powerstroke SP/ LV fluid, recommended by Ford,  are synthetic oils. These oils will withstand higher heat. The localized heat in the 6R and 10R transmission is usually worse at the converter but once the torque converter clutch is locked there is usually not a measurable change. These transmissions spend a lot of their life with the converter locked. The nature of the LV synthetic fluids make maintenance less of a factor being they normally don't need to be serviced until 100k miles. Although, if abuse is high it's always recommended to do it sooner. Early maintenance is always cheap insurance. 

    Customers running theSPE 6r140 Proprietary Solenoid Body will see a 50°F drop in transmission temps due to the patented thermoreg control valve that drops the transmission operating temperature by 50 deg F. This helps with transmission fluid breakdown.


    As always remember maintenance is key to the health of your engine and transmission. So before deciding what temps you're ok with, make sure your fluids are ok with them first.

    4 Responses

    Craig A. Hunter
    Craig A. Hunter

    August 13, 2021

    I purchased a 42’ fifth wheel toy hauler. I live in Pa. and plan to travel to Az. this winter. I figured the weight will be around 19.000 lbs. went towing. What are the high temp’s. for oil, trans., and water for a 2017 f350 dually diesel with 4:10 differential. I installed the banks super gauge and the expansion gauge. But, I don’t know what the high temperature’s are. If I new I could set my gauges to worn me things are getting hot.


    July 31, 2020

    How does the oil sample omg work? For instance, if I wanted to sample the oil on my 2019 f250 w/ 6.7l? What do I need to do? Or how do I go about it?

    Marcus Sheffield
    Marcus Sheffield

    April 19, 2020

    Always enjoy reading and learning facts from SPE. Thanks to SPE my 2018 f-250 Limited is one hell of a work or play truck.
    Thank you for your time and commitment to the diesel community.

    Jerry Birdwell
    Jerry Birdwell

    April 19, 2020

    One other thing I don’t see is oil sampling. Having your oil tested regularly had many advantages. If you’re changing your oil too often or not often enough, it will tell you. Also, will let you know if anything is wrong long before a part fails.

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