Hello,
I have a Friend who purchased a cat D6C and was questioning me about the fluids it required. 
as far as I could find out the Final drives should have TDTO 50 or SAE 50 meeting TO 4 requirements. 
transmission TDTO 30 or SAE 30
hydraulic system a quality 10wt hydraulic oil
and Engine oil just a Quality Diesel engine oil like Shell Rotella. Will use 0W40 as its cold season up here in AB canada. 
Im just wondering if this is sound information and is there any “Need to know” or critical information a guy should know about these old girls. 
thanks for any help in advance!

i think your suggestions are correct...except for the Final Drives, i was told 80-90W Gear oil or something similar....you might be OK with TDTO 50, not sure

my 941b calls for 50 weight oil in the final drives so i think you will be ok--50 wt motor oil is very close in viscosity to 80-90 wt gear oil

50W for finals is ok for colder and warm temps. 30W for engine oil or 15W-40 is ok. 10W Hyd. 30W transmission drive train for the trans/strg clutches.

I had a BAD result using 50# engine oil in final drives!! ruined a bull gear and a reduction gear, I have changed to synthetic 75-140 and in hot weather a synthetic 250 unlike mineral oil it will flow in cold weather in this day of expensive hard too find gears it’s darn cheap insurance!!!! 17a farmer

How were the bearings and gears before using 50W?

When I put the 50# oil in those were NEW CAT gears, I am using a soil 250 now had no trouble will be putting a 50# power shift oil in a D5 and an AG6 direct drive my mechanic said they will be a lot happier than with the 30# you need some body to your oil that’s why the old ones lasted so long heavier oil 17afarmer

was the oil zinc fortified. zinc fortified oils cant be used due the bull gear being made of bronze the zinc additive will attack it.
 

the easiest is contact the tech advice service of your chosen oil company they will give you a recommendation of what products in their range are suitable. some brands have different names for their products in other countries. even though the product is the same the name can be different they also change names too. chevron recently changed branding on all their heavy equipment oils and has grouped them under the delo brand name.

Never had problems with Cat 50W TDTO for years.