I just want to know which one is ok to use in my 1980 123 240d.

Dextron III ATF. Already answered.

That is not the correct stuff for the benz and will cause premature pump wear.

so we think that the w123 taxis in morocco were buying benz PSF?

Ive used regular discount PSF in my W124 - added a bit of additive to it too, same in the W210 (same dose of additive too) - works a treat - no noise no issue, infact its pretty sensible to use a synthetic ATF in there if you live in the arctic tundra - easier to pump when everything is freezing.

PS fluid in a PS pump. What is not correct? The cars are 28 years and counting, how premature is 28 years? You expect it to last another 28 years? I just do not get it.

x2. Unless all taxi drivers are oil sheikh. Even oil sheikh is hurting now with oil below $50 per barrel.

Have you ever poured the two out? I mean there is a big diff in viscosity, forget about the additives etc.

No. Do I need to???

Are they designed/formulated to be PS fluid? I just want to spend $0.99 to top up my PS reservoir. I think it does the job.

OK.

I've been a mechanic for over 40 years. I've had a fair amount experience with leaky benz power steering systems. Hoses you replace w/filter & flush. Weeping seals seem to like DexIII. It seems to stop/slow the leak. I suck the old juice out, install a new filter, flush & then top off. ATF+4 (MOPAR) seems to just disappear onto the pavement. Ditto the "Import rated/approved" stuff. DO NOT use type F (old Ford stuff) - it has "friction enhancers" (aka micro-grit). "Power Steering" fluid also seems to find the floor quickly. There is nothing wrong with MB juice but the price. I wouldn't make to much of this advice - it's worth what you paid!

IIRC, DexIII is Dyno Oil, while DexIV is synthetic, which would explain a lot of difference.

One caveat...you don't want to use ATF if the car has SLS. The reason is that the cold viscosity is too high...it causes a rough ride until it warms up. The alternative for SLS cars is a synthetic hydraulic fluid, like Pentosin 11s. But I'm not sure if there's much savings over the Mercedes stuff.

I use ATF+4. Current Chrysler cars use it and Chrysler recommends retrofitting it in older cars when doing major work. The main reason is that it eliminates many fluid chatter problems. It shouldn't hurt the seals in a 300D, but don't expect me to pay (as winmutt imagines) if you get leaks. That isn't unexpected in any 30 yr old car.

Post #25 is correct about avoiding Type F ATF. I have used it in 60's car transmissions to give firmer shifts. Some say "F" = "friction", others say "Ford". Regardless, it does have more friction modifiers, which you don't want in power steering.

The SLS fluid, at least for the older cars, is actually an obsolete aircraft hydraulic fluid. The owner's manual for my 1981 300TD has a list of acceptable fluids that includes Aeroshell Fluid 4. Your local FBO will know it as MIL-H-5606A and have it in gallon cans. ;)

This is not the same stuff you want to put in your power steering reservoir. For one thing it's rather thin compared to Dexron III.

I have heard that the seals and diaphragms in the SLS system are incompatible with ATF and will soften and fail if you use it. I did briefly have "tractor fluid" in my system and it stopped a couple minor leaks, which I suppose suggests seal swelling that could turn into degradation over time.

Actually, it's the other way around. Friction modifiers make things slipperier, for smoother shifts; Type F is notable for having *no* friction modifiers. Your conclusion is correct, though. In a power steering system you want as much lubrication as you can get.