Oil vs ATF - what is the real difference?

I am planning on getting a former bus, as in city or tour bus, and making it a motor home, and i belong to a few groups for that purpose. Some of these folks that convert the busses into motorhiomes scare me, with questions like 'what do you mean by a 'three point turn'?

Anyway, these bug critters run Allison trannies (various models) and Sheppard power steering....so the discussion of what fluid to run in them came up.

Appaently you can use ATF or 10w40 motor oil...not together fo course, but drain, flush and pick one....and off you go!

I don't think power steering it too picky on type of ATF, but i have NEVER heard of using motor oil in a hydraulic application like this before! And as to the auto trans....come on, put 10w40 in instead of Dexron???????

Anyone have any experience here or know what base these fluids run....ATF cleans, oil well, mekaes thigs oily and dirty....gotta be different from molecule 2 on up the chain, right?

Whoa Chris, hold on a moment.

Let's have some basics here. I'll contain my remarks to conventional motor and trans oils as far as the chemistry part (minor) and leave any discussion of synthetics aside for the sake of brevity.

The base oil constituent of either an ATF or a motor oil is essentially the same chemically, the only variation would be viscosity. You already indicated the motor oil you're considering is a 10w40. The equivalent viscosity on an ATF is approximately a 5w20. What that means is that the motor oil starts with a base viscosity of SAE 10w (as measured at 0 degrees F) and has additives that slow down it's rate of thinning (all oils thin as they get warmer) so that at 210 degrees it has the equivalent viscosity to an SAE 40 oil. Similarly for the ATF it starts with a base oil of SAE 5W and has additives that slow down it's rate of thinning to the equivalent of an SAE 20 at 210. So, as you can see the ATF is a fair bit thinner than the motor oil across the temperature scale.

Then there's the rest of the additive package. They each contain anti-rust, anti-foam, and anti-oxidation additives. Motor oils contain a high level of anti-wear additives (mainly for the cam shaft) and detergent-dispersants (to deal with the by products of combustion), and do not make dirt, they suspend that which results from the engine's operation. The ATF's have moderate amounts of anti-wear and dispersant, plus friction modifiers for the clutches and seal swell additives for the seals, as well as dye for identification purposes. It could be more complex than this, but just getting the basics out here.

You brought up hydraulic systems. In a temperate climate (excluding extreme hot or extreme cold) a typical hydraulic fluid will be the equivalent of an SAE 10, in fact many construction sites will use an SAE 10 motor oil in their hydraulics. Others will use Hydraulic Tractor Fluid, which more closely resembles an ATF. For colder temperature operation it's not uncommon for some to use ATF in hydraulic systems because of it's multi grade viscosity. Thinner for start up, be retainer a higher functional viscosity when up to operating temperature. The one problem with using an ATF in a hydraulic system is seal material compatibility with the seal swell additives in ATF. For instance, if you look in the owner's manuals of most cars today, they will specifically prohibit the use of ATF in their power steering systems because the seals will be degraded, causing leaks. They will specify a "special" power steering fluid, which is very similar in formulation to the ATF, but without the seal swell additives and dye.

Motor oil in manual transmissions is not that uncommon, and for that matter neither is ATF. But the automatic trans is pickier, mainly for the friction modifiers that are present in the ATF. Also the higher viscosity could have a negative affect on shift levels and smoothness. And while you probably could get away with 10w40 in the power steering system, it might lead to sluggish steering response if the system is viscosity sensitive.

I've got to believe that there are manufactuer recommendations that come with the documentation on these rigs. Rather than listen to a bunch of homeys that are guessing, I would suggest getting that documentation and following what the guys who designed it say to use.