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"M46??? Use ATF Type F, but light synthetic gear oil is preferred."
"What's this mean? Synthetic 80W90? Have you tried this?"
Actually I really only consider synthetic 75W-90 as "light". Amazing stuff....seems "thin" to the 20W-50 dino crowd, but can be used in dump truck rear ends with no problem or M46/47 trannies. I really recommend Amsoil Series 2000 75W-90 (since I SELL AND USE it), but MANY MANY folks have had great sucess with Redline MTL (which is closer to 70W-85 or so). Redline also recommend MT-90 which I think is a synthetic 80W-90. NEVER EVER PUT REGULAR PETROLEUM GEAR OIL in a M46.
"My '90 had Mobil1 10W30 installed by the PO in the M47. Shifts nice."
It'll work....it's just that you don't get the great gear protection of the "shock proofing" additives in a synthetic gear oil.
"I'm having delayed o/d in my '84 & '86. Synthetic Type F doesn't exist, and even Dino Type F is getting hard to find. I don't think Fords have used it since the mid-'80's."
I still see it. But good universal synthetic ATF such as Amsoil meets all the requirements of Type F. Lot's of folks (especially the really cold places) use Amsoil Universal synthetic ATF with great sucess.
"A respected Brick mechanic of my acquaintance uses Castrol dino 20W50 in his M46's. His prime driver has over 400k without rebuilding the o/d, and still goes into o/d nicely."
20W-50 probably protects OK....I would be a tad worried to use it in really cold places. Here's why. At ambient temperatures dino 20W-50 MOTOR OIL has a viscosity higher than 90W GEAR OIL. (It's different scales!) And at cold temps 0° and below, petroluem 20W-50 hardly flows. Amsoil 75W-90 pours at -50°F.
"Recommendations for M46's are all over the place... Syn gear oil, Redline MTL, thick dino oil, thin syn motor oil. So which is it?"
Simpler than it sounds. Here's my recommendations:
#1) "Light" SYNTHETIC gear oil if you live somewhere it does get below maybe -20° F. Either Amsoil Series 2000 gear oil or Redline MTL (Redline also says the MT-90 is OK)
#2)Amsoil Universal Synthetic ATF if you live in a very cold climate.
#3) Petroleum based Type F ATF.
#4) I stay away from motor oils for the reasons mentioned above (yes I know it'll work fine, and Volvo even recommends them, but motor oils are different than gear oils - they are specialized more and more for the guts of an IC engine, NOT a gear box) If you want to use a motor oil at least use a synthetic motor oil.
#5) Do not use Dexron ATF, petroleum gear oil or any other oil.
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