Hi Paul,
This whole thread is nothing but slips of paper really... both sides of the argument. Just to add some spice, let me add some anecdotes that I lived through... not the active participant, but his helper.
To set the stage: My dad had an IH truck that had a Borg Warner 3 speed overdrive transmission. There were two versions of the same exact transmission, one with the OD, and one without. The spec for the 3spd OD trans was SAE 90 straight mineral oil, and the spec for the 3spd trans was SAE 90 straight mineral oil, but hypoid was also acceptable. To add some further complexity, the truck had a posi-traction rear that needed SAE 90 hypoid gear oil with a friction modifier. To add a time frame, the truck was a 1962, and my dad eventually put nearly 400K miles on it.
My dad was thrifty. And quite busy. The SAE 90 straight mineral oil IH specified for the OD transmission was at that time unavailable from anyone other than the dealer (and very expensive), but SAE 90 hypoid gear oil was everywhere. The friction modifier was available only at the dealer and also very expensive.
At that point in time, you could buy a half dozen different brands of SAE 90 hypoid gear oil, from your local auto parts store. All said they superseded the SAE 90 straight mineral oil, and many said they superseded the straight mineral oil, and also provided the friction modifier needed by limited slip differentials. My dad needed (wanted ?) to service the truck, so he bought a 5 gallon can of the best stuff he could find, which happened to be Valvoline, and drained and refilled his transmission, OD unit, and rear differential. Bear in mind that this truck had about 50K miles on it, so it was essentially brand new.
Immediately after the change, he noticed that the limited slip differential behaved a little differently. It tended to go "chunk, chunk, chunk" when he went down the highway on exit ramps, and other places where the truck was making long slow sweeping turns. To fix that he went to the dealer and bought the friction modifier (that Valvoline said wasn't necessary) and that fixed the problem... for a while.
One year after the change, the overdrive unit froze up solid.
My dad took apart the overdrive unit, and found that all of the little needle bearings in the unit were ground to dust. He went to the dealer, and the parts guy said, "You put hypoid in it, didn't you?"
My dad bought the new planetary section parts for the OD unit, and a 5 gallon can of SAE90 straight mineral oil... and some more friction modifier, as the posi unit was starting to go "chunk, chunk, chunk" again.
My dad put the OD transmission back together, and once again, it was working great.
My dad was an engineer with a very mechanical bend, and it bothered him to be using an oil that had nothing in it, in this modern age. He was talking with one of the other engineers in the office, and it turned out that this guy was one of the Navy's experts in EP additives, and, in fact, had started a company that marketed little vials of Molybdenum Disulphide... about the best high pressure lubricant, and all around slippery stuff that man has ever devised... for use in cars and trucks. This guy knew that moly was the fountain-of-youth for mechanical things... Unfortunately, the market never caught the buzz, and my dad's friend's company had failed... and he had literally tons of left over stock. He gave my dad a couple of quart mayonnaise jars full of moly powder to play with (I still have them).
The "Oh oh!" alert should be going off in your brain about now.
My dad reasoned that the only time a properly specified needle bearing ever fails is due to the friction between the adjacent needle faces. And indeed, that is true. So knowing that moly was god's gift to reducing friction in sliding steel surfaces, he added a vial of the moly to the transmission oil.
You have never heard such a stream of "gosh darn-its" , "flern durn its", and "dab nab its", in your life!
The OD needle bearings may have been protected from wear for life, but the synchronizers couldn't get enough grip to do their job. That nice modern transmission became a crash box, that needed double clutching. All from adding a simple, yellow metal safe, EP additive to the oil!
He had to take the transmission apart, and scrub the moly off of everything, which is virtually impossible. I don't remember anymore if he had to buy new synchronizers, or if he got them clean enough through scrubbing.
After doing all of that, the transmission went another 300K miles, using 90W straight mineral oil from IH. After about 300K, it failed again, and was replaced with an identical unit from a '51 Nash Rambler... which then failed on my brother's watch, and was replaced by a 4 speed.
What are the lessons that come from this anecdote?
1) the one size fits all attitude is rampant in the oil industry, and always has been.
2) their one size fits all oils work most of the time, but not in every application.
3) the manufacturer usually knows what is best for their equipment.
4) engineering is a rough business, full of minor and major setbacks.
GL-5 is a compromise. It is designed to be the one oil you use in all transmissions, differentials, transfer cases, winches, etc... It has been designed to be good enough so that it doesn't appreciably wear the industry standard test transmissions, and bearings, but not so good that it ruins the synchronizer clutches, and posi-traction rear ends. It is designed to work with ball bearings, timpkin roller bearings, and other frictionless bearings, and also with sliding "bearings", like gear teeth, and uber sliding "bearings" like worm gear teeth, and hypoid gear teeth.
It does everything pretty well, but somethings it does better, and somethings worse.
Using GL-5 in everything is an exercise in mediocrity. Greatness is available for the same price, but requires a bit more room on your shelves.
-Chuck
