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What does it mean to you?

Are you the type to bring your car to the dealer for factory maintenance? How about the local Quick-Lube? Or maybe your dad taught you to maintain a vehicle at a young age. You could be the type that looks at YouTube or TikTok and does what is recommended by the person with the most followers.

One thing I can tell you is that you're likely to get multiple answers, especially when it comes to transmission maintenance!


The vehicle manufacturer cannot give you the specific time when your oil has lost its ability to do its job because of two major variables, operating environment and operator input. They have no idea how you are going to use the vehicle in the conditions you operate it in. For this reason, they give two very safe mileage specification tables based on how you use the vehicle, which is usually specified as “normal” and “severe” duty.

What is considered severe duty? Well, if you look it up you could argue that 80% of the vehicles on the road are considered severe duty. Below was sourced from Alldata:  

Severe service/duty    In addition to the normal service schedule, some vehicles require service more often. Severe service is for vehicles that are:

  1. Mainly driven in heavy city traffic in hot weather.
  2. Mainly driven on hilly or mountainous terrain.
  3. Frequently towing a trailer.
  4. Used for high-speed or competitive driving.
  5. Used for taxi, police, or delivery service.

 I don’t know about you, but my daily commute consists of heavy traffic and/or high speeds. If it is not bumper to bumper, streetlight to streetlight it is 75-80 on the interstate (where permitted legally of course). Hilly and mountainous terrain are frequent, and my truck always has a hitch ball installed, so my vehicle falls into severe duty like many others. I’m sure you’re thinking this classification leaves a lot of room for interpretation. This is why you will get so many different answers about when it’s time to service.

 In the mid-2000s some vehicle manufacturers started to market their transmissions as “lifetime fill”, and “sealed unit” as a selling feature. This means there is no service needed on the transmission for the life of the transmission.

Vehicle manufacturers only need these components to make it past the warranty period. After that, the vehicle has made it past its expected life cycle, and it has lived up to the claim of “lifetime fill”. The problem with this logic is that no fluid on the market is expected to last 100,000 miles that most warranties cover. What happens is you end up with a transmission failure shortly after the warranty coverage expires. I’d rather see the transmission last double that.    

So, what’s the right answer? You can never go wrong with servicing it sooner, however, with the price of oil and labor, maintenance can get pricy. When it comes to servicing the transmission, I will personally not go more than 50,000 miles on my transmission fluid without getting the oil sampled, regardless of the recommended maintenance schedule. A periodic inspection of the condition to see if there is a change in smell or color is recommended. This can be done while inspecting the fluid level. If the transmission fluid looks like the coffee you made this morning or if the fluid has a heavy burning smell, it must be changed. The exception to this is synthetic fluid, which smells funny when it is new.

Oil samples

In some cases, it makes sense to send oil samples to a lab and have the samples analyzed. In the lab report, they will tell you not only the life left in the oil additives but what other materials have collected in the oil. For vehicles that have larger oil capacities or use expensive type oils, this can be very cost-effective because the sample is cheap. At the time this article was written this service was between $30-$50 depending on the test used and the company doing the test. You may find you can extend service intervals safely when samples show good oil properties remaining.  

Oil sampling has also gained popularity recently for early failure detection. Oils with elevated levels of iron, brass, or aluminum may warn you of a pending failure. Below is an example from one of the companies that provides this service.

Photos sourced from Google.


Oil type is important and can be very confusing for someone who is not a chemist. During extensive conversations with multiple oil suppliers, we have found inconsistencies between the OE and aftermarket. Being careful not to violate nondisclosures I will let you know there is not much difference in base oils. The differences are in the additives used. Some additives blend well, and others simply don’t. If it looks too good to be true it usually is. Don’t be afraid to question the label on the bottle. Most of the information there is about sales and marketing.

The two most important parameters for selecting the correct fluid are the viscosity and the chemistry of the additives in the fluid. The viscosity is measured at 0, 40, and 100 degrees C, with 100 C being the most important parameter. For example, a Dexron VI fluid viscosity is typically around a maximum of 6.4 cSt (centistoke), with Dexron III being at least 7.5 cSt at 100 C but is usually much higher. The second component is chemistry. For example, a Dexron VI fluid usually has 1 – 2% added sulfur and phosphorous to get the proper application on clutch packs and the torque converter. Please note that all this information is for reference, and not designed to guide you in creating your own fluid for your vehicle. When in doubt, contact the dealership or a fluid supplier affiliated with the OE manufacturer of your transmission.

During some discovery calls we learned that one supplier had a few different fluids to cover the applications we offer, another supplier had a misprint on their application sheet and another supplier is advertising one fluid that covers everything from TH350 to 10R and 10L transmissions all coming from the same barrel. I throw the red challenge flag at this one. There isn’t a single fluid that can cover applications requiring high, low, and ultra-low viscosity. Ironically this supplier has been the tightest-lipped about what they are putting in the bottle.

When pouring the lifeblood of an automatic transmission down the funnel, trust in the fluid you’re using is paramount!

Service type

There are two types of fluid service common in the industry, and each has pros and cons.

  1. Fluid exchange - with a specialized machine connected in line you start the vehicle and old fluid is collected while new fluid is added. The machine measures the amount taken out and adds the same amount to the system.
    1. Pro - this allows all the fluid in the system to be exchanged.
    2. Con - because the pan is not removed the filter in the pan is left un-serviced.
  2. Drain and fill - remove the pan and let the fluid drain from the case, replace the filter, reassemble, and refill the transmission.
    1. Pro – the filter is being replaced and the pan is being cleaned of contaminants. This also allows you to see any debris that has been collected in the pan.
    2. Con – fluid in the torque converter, clutches, and valve body will not be exchanged, roughly two-thirds of the total fluid capacity.   


We could write an entire article about filtration. For this article, we will keep to when to change and the filter types used.

  1. Types of filtering
    1. Metal screen – used to filter heavy contaminants from reaching the pump. In most cases, this style is used with an external mode of filtering the smaller material. This style filter may be cleaned and reused.
    2. Media – uses a mesh material to filter contaminants down to the smallest microns protecting components like the VB that have tight clearances. This style filter should be replaced when servicing your fluid. Do not attempt to clean this filter.
  1. Locations
    1. Internal – somewhere inside the pan or between the transmission case halves if there is no pan. *If there is no pan the filter is non-serviceable without a complete teardown
    2. External – some systems use a filter that is serviceable without removing parts of the transmission. These filters most likely are used when there is a metal screen-type filter inside the pan.
  2. Mount style
    1. Spin-on – looks like a traditional spin-on engine oil filter and it may be located externally or inside the pan.
    2. In-line - sometimes there is a filter cartridge connected to rubber hoses between the transmission and the cooler. It may also be a replaceable filter media cartridge in a metal housing that comes apart.
    3. Sump – is the traditional filter in the pan that is connected to the pump or valve body and picks fluid up from the bottom of the transmission pan.


There is a lot to consider in prolonging the life of your transmission and we hope this article was both eye-opening and informative of the mysteries and misinformation surrounding transmission maintenance.

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