shell 134 is usable in place of 3403 according to the tech sheet of 134 and also according to a data sheet from MB themselves - I had it somewhere on my garage PC.

shell 3403 is unicorn blood in the USA - however it performs the absolute best in these transmissions. It behaves a bit odd in its life - it goes from being blood red to green to yellow then brown like chrysler ATF+4 does.

the only MB listing fluid you will find today for these NAG1 gearboxes in USA is the shell 134 ATF. I sometimes come to know of some oil depot having old stock 3403 ATF but never have actually seen it or been able to buy it. Its called shell spirax 3403M115 now.

According to some transmission rebuilders I have met who work on these - the old shell unicorn blood is nothing special now - it was super duper special when this gearbox was launched as such electric gizmo gearboxes were not common place, most autos were oldschool or a slow move into some electrification - The NAG1 was light years ahead. Such ATF is pretty common now and very closely mimics other manufacturer spec ATFs.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)