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.
