The Stovebolt Site Forums The Shop Area Driveline transmission lubricant

I have read several posts to NOT use hypoid oil in a synchro transmission because it is "too slippery" but how about the 4 speed crashbox?? is "slippery" a good thing here?

The issue with hypoid oil (the current GL-5) is that it contains sulfur compounds that will eat up the brass synchronizer rings in manual transmissions. It's not that it's too slippery. You can use a GL-4 gear oil or straight mineral oil and your synchronizers will be OK. If the crashbox has no synchronizers, you could use anything in it, including GL-5 gear lube.

Interesting. I was told back in the day that the sulphur compounds were a friction modifier that allowed the brass to grab the polished steel to create the synchronizing effect. Maybe I was told wrong, although it was the owner of Salt Lake's largest manual transmission specific overhaul shop at the time (mid 1990s).

GL5 contains EP additives that attack yellow metals such as synchronizers. I use synthetic transmission oil on all of my transmissions. It does reduce the synchronizing effort of the synchronizers. But that doesn't concern me. I'm accustomed to "floating" the gears when I shift. Synthetic would be just fine for an old spur type transmission because it doesn't have synchronizers. And the nice thing about synthetic is that you don't have to change it for many years.

Does synthetic resist the shear effect chopping the long chain polymers into little pieces? I thought gear oils were formulated to resist the shear effect of gears and to resist foaming.

However, the Spicer/Brown-Lipe brownie I have says on the info plate to use SAE 50 engine oil, so I use 20/50. It's a crash box.

Why do you use 20w -50 instead of the straight 50 as specified?

Because I run it in the winter and it shift better when cold. Plus, 20/50 is readily available and straight 50 is not always in stock.

An add on to my transmission oil comment upthread, I recently switched back to GL4 gear lube from NAPA's very spendy SAE 90 mineral oil. I changed because the three-4 shift grinds when the gearbox is hot, like after several hours on the road. I bought Red Line MT-90 which about the same price as NAPA's and is a 75/90 oil listed as yellow-metal safe for synchronized, manual transmissions and transaxles. By the time I got out of the yard I could tell the difference. The SM420 is much quieter in 2nd and 3rd and shifts smoother, at least in hundred-degree weather over the first five miles. I'm taking the truck on a long trip, loaded, in hot weather and I'll let you know how it does under those conditions.