There is a lot of factors to determine whether single or 2 tanks is better for YOUR car. 2 tanks is always preferred if you do not mind the complexity and you value your car.

1) How long you want to keep the car?
2) How many miles on it already?
3) How deep is your pocket?
4) How much you paid for the car?
5) Home-brewed ( designed it yourself ) single or dual tank system or from vendors?

Home-brewed 1 tank can be had for < $150. 2 tanks system is much more. System from vendors varies but you won't see a lot of change for $1K for the cheapest one.

This is what I would recommend. If you use 1 tank, make sure to put after-glow circuit in to make the WVO burns better during cold start. Filter, filter and filter your oil. Install a diesel water separator to de-water your WVO.

I have had good results with my home-brewed 1 tank conversion. The after glow helps a lot and you can feel the engine idles much better during cold start. I am in SoCal with a garage, so the coldest morning may still be above freezing.

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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.