At one time, "W" stood for "winter". For instance, a 20W-20 motor oil was tested for flow rate at 0 degrees C. (32F) and again at 100 degrees C. (212F). A 10W-30 oil has the same flow characteristics as a SAE 10 oil at the cold rating, and lubricates like a SAE 30 oil when hot. Its physical thickness DOES NOT change with temperature, only the lubricating capability.
85W-140 gear lube flows like SAE 85 oil when cold, and has the lubricating capability of SAE 140 when hot. Apparently, the "600W" does not follow the old SAE rating system, which involved timing the flow of oil through a fixed-size orfice at various tempertures. Faster-flowing oils got a lower numerical rating- - - -SAE 10 flows fast, 30 is slower, and 140 is VERY slow!
Jerry
