Understanding CVT Fluid Viscosity and When to Replace: Expert Insights

[Ephant]

I went to Subaru for period maintenance checkup. They told me the car was due for CVT fluid replacement which was done every 5 years or 40,000 miles. It's already 7 years even if the mileage is only below 20,000 miles so I let them go ahead. The service advisor told me as time passes, the viscosity of the fluid can become thick. How true is this?

After I got the car service. I tried to research in the entire internet. I read some car brands even claim the fluid (CVT or AFT) is lifetime. Many doesn't replace it until 80,000 miles. What is the truth about the so called viscosity? How can CVT fluid or ATF (Automatic Transimission Fluid) viscosity degrade over time. What is the chemistry involved? My car has no warranty anymore so I don't know if I made the right decision to get it replaced for $300 at only less than 20,000 miles but 7 years already. My friends told me I was scammed. They didn't change their ATF even after 80,000 miles.

 

[DrJohn]

If the oil molecules contain double bonds between carbons i.e. C=C then cross linking might occur, which results in bigger molecules, and viscosity will increase. In rubber cross-linking is caused by exposure to uv light, causing little cracks to be visible in tires.

However, if it was a modern synthetic oil, I would expect them to avoid using oils like that, to be honest.

So I think the garage just followed what the manual said, and never questioned how likely a failure was. But I am not an expert on cars.

Garages do tell you scary stories about things going dramatically wrong if you don't change certain things. Like cam belts. One of my cars had a cam belt change at around 60,000 miles before I bought it, it was six years old. I bought it with100,000 miles on the clock, and by the time it got within 400 miles of ten times round the planet (248,150 miles on the clock! And now 19 years old.) I still hadn't changed that cam belt. I was very keen to get that ten times round the world on the clock but it developed a problem that made it a bit unsafe to drive. Recommended change interval was every 60,000 miles.

 

[berkeman]

Wow, your Subaru has a CVT? That's amazing; I didn't know that any production cars had CVTs in them. What model do you have? That must be pretty cool to drive...

 

[jack action]

I didn't know that any production cars had CVTs in them.

Well, you will be amazed by this Wikipedia page ...

 

[berkeman]

Well, you will be amazed by this Wikipedia page ...

Hey, waidaminute... Are these all electric vehicles?

Chevrolet Volt

Or did most ICE vehicles change to CVTs while I was sleeping in my cave...?

 

[jrmichler]

CVT's are mostly used in smaller vehicles. Their mechanical efficiency is lower than gear transmissions, but the ability to run the engine closer to its best efficiency point more than makes up for the lower efficiency. My wife's 2021 Buick Encore GX has a CVT. It has good performance, and gets excellent gas mileage.

These transmission use special traction drive fluids (search the term). These fluids experience increases in viscosity under high pressure. One (not the only) manufacturer of these fluids is Santolubes, which manufactures a line of traction drive fluids using the trade name of SantoTrac Traction Lubricants: https://www.santolubes.com/products/santotrac/. I remember this particular product from a study of traction drive transmission that I did back about 1977. An excerpt from their web site:

These lubricants, ordinarily liquid, under extreme pressures in the contact zone form solid films which keep surfaces with potential metal-to-metal contact apart, as well as provide traction. This results in increased speeds, enhanced power output, wear prevention, and extended useful life of the equipment. Traction is measured by coefficient of traction, which is the ratio of transmitted force to contact force. Traction coefficient of SantoTrac fluids at 40℃ and 5-10% slide-to-roll ratio is in excess of 0.1. While these lubricants are fluids of choice for many applications involving elasto-hydrodynamic lubrication, such as gear systems, ball and roller bearings, rolling assemblies, and belt drives, they are especially useful in extending the life of the industrial gear-less Planetroll type systems and are also the choice lubricants for the new generation automotive transmission systems, where continuously variable transmissions (CVT) are used.

 

[Ephant]

I researched and found out that Subara type II CVTF fluid is made by Idemitsu. I went to this website for the exact fluid they used. Do you have any rough ideas what ingredients they could have used? Does it contain any viscosity expanding components like double bonds between carbons i.e. C=C where cross linking might occur, or others that can thicken the viscosity in time? If we can't know the ingredients used, maybe there are other brands of CVTF fluid where they are known?

 

[Tom.G]

As a partial answer to your question about viscosity, see their web page:
https://www.idemitsulubricants.com/product/cvtf-type-sb2

Your best bet for additional information is to gets the Material Safety Data Sheet (SDS) from the manufacturer.

I tried that but they require you have a Business account with them, in fact, they state that a Guest account Will Not give you access to the SDS. I was under the impression that applicable law indicates that the SDS be freely available.

Anyhow, here is a link to the SDS in PDF format:
https://www.idemitsulubricants.com/dA/d65e4c8145/sds_idemitsu cvtf sb2.pdf

Page 2 indicates: Lubricating Base Stock, 80% - 90%
Page 4 references: Oil mist, mineral. Also lists Personal Protective Equipment
Page 6 references: Petroleum-based (mineral) oils...

So that leaves 10% - 20% of other unstated/unknown additives.

Cheers,
Tom