First post, so please bear that in mind. My background's similar to Cat 793 - large open pit mining tools. Currently working for a mining company on an open-pit gold mine in the Caribbean, but for almost 20 years I worked for Cat dealers, and before that for other mining companies. Started back in the 70's on open-pit coal. We always joke that if you can't walk underneath it standing up it it's not worth working on ...
A few thoughts about oil......
Most manufacturers will give you options about what oils you can use in a certain compartment. The idea is to be able to reduce lube inventories by stocking less different grades while still covering your bases regarding compliance with the OEM oil specification.
For example in a current model D10T Tractor you have the option of using 9 (yes, that's right NINE) completely different types of oil in just the hydraulic system. Some are Cat's own oils, changed at 4000 hour intervals, others are commercial oils changed at 2000-hour intervals. A number of the oil options have direct relevance to the ambient temperature, and obviously you wouldn't use the same viscosity of oil in Northern Canada as you would in the desert in Western Australia. The list includes synthetics & mineral-based oils, specific hydraulic oils, engine oils, TDTO (TO-4) oils, & multi-purpose tractor oils. ALL are acceptable provided they fit the ambient temperature limits listed in the manual.
However for 24/7 mining operations we usually toss the ambient temperature recommendations out of the window, basically because they're designed for cold starting every morning and or course machines that work 24/7 allegedly never get cold. Even if they do it's generally the mechanics and not the operators who start them from cold and thus we can control how they are warmed up to make sure components don't get damaged. So as a rule we would go for an oil viscosity that might be heavier than suggested by the ambient temperature chart for the machine in question.
A point was made about oil-cooled brakes on Cat trucks. We have 777F & 789C both with oil-cooled brakes. This system is combined with the converter and dump body hoist. It needs a TDTO oil (or commercial TO-4) with friction modifiers because of the brake packs. We use a commercial SAE 10W TO-4 in these systems. However the Steering systems on the same trucks can get by with an SAE 10W hydraulic oil. Rather than have two 10W oils on site we opted to run all hydraulic systems on 10W TO-4 for ease of inventory and also to make sure that someone didn't dump a pure hydraulic oil in a brake cooling tank by mistake. Consequences would have been pretty dire.
Truck and Tractor final drives we run SAE60 FDAO oil (FD-1). It's really the only option for 24/7 mining operations as opposed to an SAE50 TO-4 which will not maintain a good enough lubrication film under high stress conditions. Weird thing we went from SAE50 to SAE30 TO-4 in motor grader tandems because the 50 was giving us brake issues.
A digression to Hitachi. I've seen a number of users switch from the Hitachi zinc-free Super EX HN 46 to other brands of hydraulic oil (non zinc-free) and go through total pain with pump failures. Until recently the Hitachi oil was the only zinc-free option on the market but now I hear there are others. One such option is Conoco Ecoterra HVI 46, but I have no experience of that oil. We're running 2 Hitachi EX-3600 machines here in addition to our Cat fleet.
One last thing that a lot of people forget is about oil cleanliness. If you can keep oil clean using regular oil sampling and a particle count to measure the ISO Code, and dialysis (kidney looping) when necessary you might be amazed at the increase in life you can get from components. The last decade has seen pretty much all the major manufacturers put in big efforts as regards contamination control.
