There is no problem using R134a.

1) Fill the compressor with 8oz of Ether oil. Pag oil is OK but it is better with Ether as the system was using R12 before and has mineral oil. Ether Oil mixes better with the old oil unless you can purge the mineral oil completely.

2) Put 2 oz of oil in the rec/drier. It is not necessary but some people recommend it.
3) Pull vacuum down to 20-25 hg or better and make sure it holds for a few hours, or overnight.
4) Charge with R134a.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.