One of the top priorities of automotive engineers today is to make vehicles that require less maintenance. In this they have done an excellent job making cars last a long time without all the routine visits to the mechanic for nothing more than preventative maintenance. When a vehicle requires less maintenance it is cheaper for the owner, and less of a hassle. The auto manufacturers know that people want to buy cars that require less maintenance so they have engineered them this way.
Spark plug changes are only due every 100k miles or so, drive belts last a very long time, and even the old oil change really doesn't need to be performed as often as it used to be. The modern automobile is the most trouble free, and maintenance free contraption ever built by the hand of man. Heavy-duty trucks, heavy equipment, and other similar vehicles of the modern era are not as trouble free, and maintenance free as the automobile.
Lately however, it seems as if engineers have taken things a little too far when it comes to reducing preventative maintenance. Now, it is very common for a vehicle to have lifetime fluids. This means that some of the fluids on some vehicles are said to be good for the life of the vehicle. One of the most common lifetime fluids is transmission fluid.
With advanced technology in the processing of petroleum products we now have many types of automotive fluids that are labeled synthetic. This title usually refers to the way they are produced. Advanced processing methods give the end product better properties for performing the function for which it was designed. One of the abilities of these new synthetic fluids is greater longevity in the gear box or other mechanical assembly where it works to reduce friction and condition parts. These fluids do last longer.
The term lifetime in reference to fluids is somewhat misleading, and really rather vague. What is the definition of lifetime? Are they talking about the life of the vehicle, the life of the owner, or maybe just the life of the warranty? Maybe they mean the lifetime of the fluid itself. As if to say the fluid has a lifetime, and they are proposing that the fluid doesn't need to be changed until its life has run out. That would be very misleading.
The other thing that is a bit beguiling is many times forever fluid only applies to vehicles that follow the normal maintenance schedule. Most vehicles still use a normal and a severe maintenance schedule. The idea is to follow the one that applies to you. Of course based on the definition of each schedule just about everyone is severe.
The problem with lifetime fluid in this case is the normal schedule calls many fluids lifetime fill. So if you live in Florida and never drive too fast, or too slow, or too little, or too much, then you never have to replace the transmission fluid, or the differential fluid, or the power steering fluid. If you are severe like 99% of drivers, the fluid must be changed according to the recommended interval.
Some vehicles literally do not give any recommendation on the service interval of their lifetime fluids. The engineers literally mean to say that the fluid never wears out. This of course cannot be true but to the original owner of the vehicle who keeps it for 10 years or 120k miles they really do not need to worry about fluid service.
So, if you purchase a vehicle with transmission fluid that is lifetime fill, when do you need to possibly have the fluid serviced anyway? A reasonable interval for servicing just about any type of alleged lifetime fluid is about 100k miles. This is not really all that bad because it might mean that the average used car buyer still has to only service the fluids of this vehicle maybe once in the time they own it. Automotive engineers have done a good job of reducing maintenance costs on cars, but they will never eliminate the need for fluid service.