Having Panic Attack.....

[bostondiesel]

In my recently acquired from ex-husband 83 Mercedes 300d, I am having a panic attack. He took care of all the maintenance, now it is up to me. I don't do the work my self, but have a trusty mechanically inclined person (who really only knows VW's), help me. My anxiety lies in doing the transmission service. I have read and asked and reread and asked again the parts I need, I have read up on the forums on what to use, etc. Am so nervous that doing the service will somehow make the old girl run badly. I am talking from experience with a Dodge Durango, had transmission serviced and it crapped out. I have read on the threads to use synthetic Dexron, but when I went to get the parts today they didn't have this, only Valvoline DEX/MERC ATF, that the parts guy assured me is great. Is this good to use in place of the Dexron III? and for doing the rear differential fluid change my owner's manual suggests hypoid gear oil SAE 90, or 85W90. Parts guy looked it up in his manual and gave me Castrol Syntec Gear Oil SAE 75W90, said the difference between 75 and 80 is negligible. Any words to soothe my fears? Did I get the right stuff? I picked up 10 quarts of the Valvoline ATF, or rather 9 and one non-synthetic store brand, he said mixing one in would not matter. I bought so many because I am doing the power steering filter and fluid and it calls for ATF in manual. You know the old motto, if it ain't broke don't fix it? Am I opening up any cans of worms, or am I on the right road to making her last forever? Sorry to be a pain, but I am new to all this and want to do the right thing, I love, love, love this car!