car stat: 1987 260E 2.6L M103

Hello, I wonder what common transmission fluid is acceptable for the older W124? By common I meant the ATF that you can just buy anywhere, like Walmart, Canadian Tires etc. You know the non-dealer stuffs. My transmission is slightly leaking so I figure whatever I put in to top up will just leak out during driving anyway, so why put the expensive dealers stuffs inside.

Can I just use a good quality Dexron*III/Mercon** from Walmart?

What does it means when the bottle says something like Dexron*III(H specification) ?

Thanks