HEY PAUL . SPENCER IS RIGHT ON , ON THE GEAR OIL VS ENG OIL IN THE PUP TRANS , SEEING AS HOW YOU HAVE THE TRANS APART, NOTICE THE CENTER AND FRT BEARING ,NOTICE THE SEALS, SOME OF THE BEARING HAVE SEALS ON BOTH SIDES, 90 WEIGHT CANNOT GET IN THE SIDE BEARING TO LUBE IT PROPERLY, MAKE CERTAIN YOUR BEARING HAVE NO VERTICLE PLAY AND USE 5-30-10-30 , ANY STRAIGHT WEIGHT OIL SUCH AS 30-W WILL WORK OK BUT WHEN IT GETS COLD IT WILL BE HARDER TO SHIFT, , 80-W OR HIGHER , IT WILL BE REALLY HARD TO SHIFT WHEN COLD , WE TRIED MANY DIFF OILS HERE IN ATLANTA IN THE WINTER IN THE EARLY 80,S AND COMING OUT OF FIRST INTO SECOND , THEY WOULD NOT SHIFT , THE OIL IS SO THICK THAT THE SYNCHRO RING CANNOT BE FORCED AGAINS THE NEXT GEAR , UNTILL IT GETS HOT , IF YOU PULL ALL THE SEALS FROM ALL THE BEARING S YOU CAN RUN A HEAVIER OIL, BUT THE SHIFT PROB, IS STILL THERE , I HAVE SEEN THE MUA WITH 90 W GO 5000 MILE AND BE MAKING A HORIABLE NOISE , THE BEARING CLEARENCES ARE USUALY PRETTY CLOSE AND THE THICK OIL HAS A TOUGH TIME GETTTING TO THE BEARING SURFACES BEFORE DAMAGE OCCURES, BUT I HAVE ALSO SEEN A FEW OF THE MSG TRANS WHICH IS WHAT YOU HAVE , WHEN THEY GO 70 + THOUS MI, ON THEM CHANGED OVER TO A HEAVY WEIGHT OIL AND THEY SEEM TO RUN FOR A LONG TIME , BUT WITH NEW BEARINGS , I WOULD STAY WITH THE LIGHTER OIL EVEN IN 90 DEGREE WEATHER, THE SHOP MANUAL ONLY CALLS FOR 30. WEIGHT, ON YOUR TRANS WITH A LOT OF MILES I WOULD USE THE 5 OR 10 W 30 AND ADD A BOTTLE OF THE LIMITED SLIP ADDITIVE THAT GM PRODUCES , IT WILL MAKE THE OIL CLING TO THE GEARS AND THE BEARING SURFACES , IT HELPS KEEP THE OIL FROM SLINGING OFF THE SPINING GEARS , THE PT NO FOR THIS OIL IS 10523580 GM NO, ONE BOTTLE IS ALL YOU WILL NEED , IT IS MADE FOR THE LSD UNITS BUT WORKS GOOD AS AN OIL ADDITIVE , I SELL THIS STUFF BY THE BOX LOAD . JERRY