What oil is best to use in a late 40's D4 hydralic system? No info in the maintenance manual I have. 

My 40 hydraulic scraper manual says to use crankcase oil. I use John Deere hydraulic / transmission oil in the D4 hydraulics and scraper now. Never had any seal or pump problems.

You don't say what brand/ model of hydraulic system you have, if it is a Caterpillar # 44, form 31677-1, Operations and Maintenance Instructions, says to use "Crankcase Lubricating Oil" in compliance with specifications. Above 10* f, it says use S.A.E. No. 10W.
If you have a 4A or 4S Bulldozer the total capacity is 8 US gallons (44 Hydraulic control is 3 1/2 gallons and the 4A/S system is 4 1/2 US gallons).

ISO 32 Hydraulic oil/fluid is my preferred usage.

you can use the same oil as the engine. or do what i do and use universal tractor and harvester driveline oil. the product i use is caltex textrans tdh it is made for the trans diferential and hydraulics of most tractors and specifically for caseIH/austoft harvesters and infield transporters. it has cat TO4 approvals. and that is the caltex recconmendation delo gold 15w40 or textrans tdh which is also suitable for the transmission.

I switched to Betty Crocker vegetable oil, if i blow a hose it wont hurt the environment and the used oil is for frying fish.

Rome wrote:

I switched to Betty Crocker vegetable oil, if i blow a hose it wont hurt the environment and the used oil is for frying fish.

Good one Rome!!

 

Rome wrote:

I switched to Betty Crocker vegetable oil, if i blow a hose it wont hurt the environment and the used oil is for frying fish.

Hahaha 

So l guess if you don't follow Cat recommendations you can use whatever you want. I would use the fryer oil that cooked the fish then when you're working you could smell the aroma when everything heated up 

 

Here in the diesel injection business (non CAT) we still occasionally see someone who uses veggie oil as their fuel. It smells good when burning but isn't good for the fuel system. Used veggie oil is particularly bad because of the salts and unfiltered parts of stuff. We have seen more than one injection pump absolutely trashed for this reason. I expect hydraulic systems could suffer a similar fate, but I don't have any experience with that. Elton