I'd like to toss in a little info for those who like to expand what they know, not looking to put anyone down.
That W thing seems to steer people around a little. The 600W nomenclature dates back more than 100 years, and as noted in a couple previous posts it's origins were as a steam cylinder oil. It also found good use in worm gears. The common element there is lubricating sliding action. Back then that was assisted by the use of whale oil as it was discovered to have a highly polar (think like a magnet) molecular structure. So it stuck well. Since the use of whale oil is now verboten other similar acting chemicals are employed. The W on 600 grade meant there was whale oil added, has nothing to do with "weight", or viscosity. This is not to be confused with the W designation in multi-grade oils. That W stands for winter, not weight as some may believe. Multigrade oils are graded for low temperature flow properties (thus winter), whereas straight grades (e.g. SAE 30) are not. There's more to that story, but who knows if there's sufficient interest.
Oil companies put out lots of info if you know to look for it, for lubricants it's often beneficial to read these data sheets.........solves a lot of misinformation. As noted above, one of us was told that today's 600w isn't for steam cylinders anymore. Whoever told him that was misinformed. Here's a link to a data sheet for Mobil 600w, clearly identified for that application.
http://www.mobil.com/Canada-English/Lubes/PDS/IOCAENINDMOMobil_600W_Cylinder_Oil.asp Though we may not be too familiar with that application in our day to day lives, a reasonable number of industries world wide still do.
As for what other gear oil viscosity grade the 600w most resembles the data sheet can answer that question too. Viscosity grading is based on rate of flow. Modern standards measure that in a unit of measure called a centistoke, abreviated as cST. Since temperature affects that rate of flow, the measure is always reported at a specified temperature. So, if you look at the data sheet for 600w, it measures 27 centistokes at 100c (aprox 212f for those who don't care for metric). Here's the data sheet for Mobil HD gear lube;
http://www.mobil.com/Australia-English/Lubes/PDS/GLXXENCVLMOMobilube_HD_80W-90_85W-140.asp
If you look at the 85w140 grade at 100c you'll see it measures 25.3 centistokes, close enough to be the "same" as 600w at that temp. The sharp of eye might also have caught the difference at 40c that may or may not be meaningful to you.
BTW, I'm not pushing Mobil products, they just have good info that's easily accessed. Hope some of you are still awake by this point in the post.
