Is it OK to use Dexron II in a 87 420 SEL transmission, or should I go with a synthetic ?

 

Kalan,

Personally, I would buy the exact Mercedes brand fluid, and from the dealer.

I have good reasons for saying this. The chemical composition of the seals used in gearcases and many other applications are matched to the type of fluid to be used. Mixing or using the improper fluid can damage your trasmission.

Here's why: Synthetic oils and fluids have benefits, but have the tendency to dissolve and distort, Silicone, Neoprene, and Buna seals not disigned for use with synthetics.

On the other hand, organic fluids used in a gearcase designed for synthetics is also bad. Organics break down and form acids which attack the very seals which would have otherwise been fine such as polyurethane and EPDM. In addition, higher stability may be necessary for the life of the gearcase.

The only seals which accomadate both synthetic and organic fluids are teflon, viton, and Kalrez. But these seals do not work as well around moving parts because they are stiff and better suited for non-moving applications, and are also very expensive.

You should keep this in mind when replacing seals also. Saving 10cents by buying seals at the hardware store or parts depot may be the WRONG thing to do.

So when you read that your engine or transmission is designed for synthetic, or organic fluid, type 1 or type 11, what the author is really telling you is what type of seals have been used, and you should not risk damaging them with uncertain fluids. This goes for brakes as well.

That's all I have to say on it.
Charles