It is a long answer.
The AW36 is an ISO grade that correlates to the weight of the oil. Common ISO grades for use with hydraulic systems include 32, 46 and 68. Sometimes 10, or 10 weight is mentioned in older tractor manuals as well. 10W-40 automotive engine may be used in an emergency, providing it does not contain detergent additives, and these additives used in engine oil can cause corrosion and cavitation. This occurs because the detergents can cause water that has condensed into the system to emulsify, preventing it from being separated from the oil. More about additives in motor oil in last paragraph.
The 134D is a grade or specification of performance. The grade specification
provides extreme pressure protection for maximum gear life and capacity for wet brakes and P.T.O. clutches. Wet brake noise suppression and filterability characteristics along with rust and corrosion inhibitors, water tolerance, and temperature range performance.
You can go to Citgo and obtain their Mystic Power Lubricant and it meets or exceeds the following New Holland specifications, as does the 5 gallon inexpensive UTF at Costco, nearly all the Universal Tractor Fluids (UTF), and the store brand tractor fluid at WalMart - just read the specification label looking for the specifications met or exceeded:
M-1127B
ESN-M2C134-D
ESN-M2C134-D, C
ESN-M2C53-B, A
ESN-M2C86-C, B
Save yourself a lot of label reading an just look for the words to the effect, "Meets the performance of John Deere Specification J20C or JDM J20C." Highest quality tractor fluid specification of all tractor manufacturers.
You will also hear that 10 or 10-30 or other multi weight motor oils can be used. Not factual for about 20 years, because the additive packages used to stop oxidation of modern motor oil bonds to the oil bath brakes, PTO clutches, and any other frictions in the tractor transmission, and will lead to slippage, grabbing, or failure.