Fantastic Info Nik, thanks so much for looking into that I tried many sources to find the GL rating. I'll probably end up getting the MT90 then.

+1 for GL4
FWIW
I have used Amsoil GL4 75W/90 for several years now in my S54 box.
No probs whatsoever, with smooth shifting in all gears. Easy shifting when cold, as well as hot. 
http://www.syntheticwareh.se.com/synthetic_gl4.htm
 
For easy trans oil fill from underneath I use one of these suction pumps:
 http://www.ebay.com.au/it...UN-500cc-/191634169500

Has anyone found a reliable source for synchros? Im looking for 3rd and 4th for a 1990 E153 and having some issues with parts listed as being discontinued.

Amayama and megazip have them (cheap).
Recently bought syncro 2 (336720060) 3 & 4 (same part number 3336820040) from amayama. Amayama UAE had them cheaper ($30 vs $55 out of Japan) higher postage from UAE but I got bulk parts, took about four weeks in total to receive. Before you buy ensure you have your gearbox open for if you have single or double syncro, as well as what other parts you may need.
I am about to order other parts, because once you open the gearbox there are more parts that need to be replaced than just syncro: seals, snap rings, keys, screw barrel retainers, bearings, and other bits you come across like input boot dust cover etc.
Parts are available however have superseded part numbers - got them from the help of KO Racing USA.

Stuka, check the part numbers applicable for your series of e153 (single or dual syncro). There are write ups online. Your own risk for buying parts.