Swapped to synthetic gear oil in my 18-speed and holy torque difference

For 15 years I ran the standard 80w-90 in every big rig I touched. Switched to a full synthetic 75w-90 about 6 months back after a driver complained about hard shifting in cold Ohio mornings. First cold start the gears slid right into place, no grinding. Has anyone else noticed less heat buildup on long grades with synthetics?

2 comments

2 Comments

barbara_kim11d ago

@emmajackson brings up a good point about the leaks, I actually heard a mechanic say the same thing about thinner synthetic finding its way out of old seals easier. But I've seen the opposite with heat buildup though. A friend of mine runs a fleet of Paccar engines and he switched all of them to synthetic after a test showed his oil temps dropped almost 15 degrees on the long pull up I-70 in Colorado. I've heard some drivers say it's just the placebo effect, but my own logs showed a consistent 0.3 mpg bump after the first month, so something is definitely different.

emmajackson11d ago

Oh, I gotta push back on that a little bit. In my experience, the synthetic stuff drains out easier through worn seals and can cause leaks where old gear oil never did. I also found the shifting difference is mostly placebo after the first few thousand miles, especially once the trans warms up. Take it from someone who's had to top off a leaking diff twice because the synthetic was too thin for my older setup.