Trans/gear oil question?

[BHD]

on older equipment most transmission and gear box oils are a 80/90wt or 95/140wt or some where in there, the thick goo oils,
(I am guessing here, but to get the pressure rating needed they used thicker oils to keep a oil film in place).

on most modern equipment it is a Hydraulic transmission oil that is a basically a 10wt oil, and from what I can see, does as good or a greater job of protection than the heavy wt oils, (and to my understanding now they use additives that have made a thin oil have the same or greater pressure resistance and stays in place),
even modern stick transmissions in pickup truck use the ligher oils in many of them,

yes some modern gear boxes still have the heavy oils in them or recommend them, such as combine wobble boxes that run the sickle,

but in things like post hole diggers and other applications is substituting with the modern oil a OK thing, (DID NOT SAY MIXING),

what are your opinions, and do you have any facts to back up your opinions,

I know follow your manual, but when some of this stuff was designed before the product was created,

just a question I came up with to day, when filling up the post digger gear box,

 

[Just Bill]

I know follow your manual,

Hello
You really answered your own question,
A lot of new equipment still uses the heavier oils for proper lubrication.
Run what you want, but expect more noise and more leaks if you use thinner oils.

 

[bp fick]

The synthetic gear oils now introduce a new dimension, however.
For example, when I got my RFM from Ken Sweet, he recommended that when it arrived, suck out the 80w-90 from the gear box and replace it with 75w-90 synthetic. He recommends this to all his clients. It was my intention to do do, even before he mentioned it. Kubota, for example, allows EITHER light hydro fluid or gear lube in the front axle, right in the manual. At the 50 hour service, I drain the factory fill of hydro fluid and replace with 75w-90 synthetic.

Our local farm/home store sells synthetic gear lube at the same price as dino. So there is no burden financially to use a product that will have much better flow rates, while at the same time, affording great protection in our cold winters and hot summers.

We used to use 10w-40 in our cars. Now? It's 0w-30, 5w-20. Yet, these car engines run twice as long as engines ran years ago.

The manual transmission on my pickup now calls for ATF, no gear lube. That's a change from by-gone days of 90w. Thinner, yet highly protective, fluids in the auto world is mostly do to friction and drag reduction to assist with fleet mileage requirements. Every 10th of mpg here or there adds up.

B&S lawn mowers all used to take straight 30w. Not anymore. Most now recommend a multi-vis. Times are changing.

All I know is that the lubricating/anti-wear properties of the modern synthetic fluids/oils/lube at the molecular level is far greater than the old timey dino technology of 30 or 40 years ago.