I suspect MB Medic thought you were going to LOOSEN the crankshaft bolt. You do not need any special "hold-down" tool if you're merely trying to rotate the engine in order to locate the converter drain plug. As viewed from the front, you'll turn the engine CLOCKWISE until the converter drain plug is positioned for draining.
As far as brand name goes, consult the literature that came with your MB. Any major brand of Dexron-III will work just fine. Be careful not to overfill.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.