transmission oil change - Ford XT-M5-QS Full Synthetic or MT-90?

For what it's worth:

Just changed my stock 6spd MT (RA60F) transmission oil. Put in Lucas

$30 75W-90 Synthetic Gear Lube - 1 Quart (Pack of 2)

75W-90 synthetic GL-5, went for a fairly violent 10-15 mile test drive w/ hard accelerations, hitting the speed limiter on the freeway (~108 mph or so.) Transmission shifts and operates normally so far, seems like gear shifts are slightly smoother and the tranny is a little bit quieter.

Temperate climate zone w/ mild winters, seldom ever hits 25ºF and most days are in the low 50's.

 

this is mostly a cold weather issue from what I gather(and in my experience).Prior to the swap to GL4, I’d need to pull back into 2nd with an extreme amount of force for at least the first 10-15 mins driving in the cold and it wouldnt necessarily take freezing temperatures to get there….under 50F would do it. It’s been relatively warm since I swapped out for GL4 (Ford) but some mornings in the upper 30’s-40’s and shifting is improved and just better in general. Can’t speak to the V6, 6spd guys but anyone noticing this in their 2.7 manual trans, this helps….big time. Sounds like GL5 is bad (long term) for the syncros at any rate.

 

Hmm. I should have done more homework before putting GL5 in the 6-speed manual tranny. Even though GL5 has more wear-reducing additives, apparently the additives tend to react badly with copper & bronze (the synchros,) which is why Toyota specifies GL5 for diffs & transfer cases with their all-steel gears -- but GL4 for the manual transmission.

As soon as I can get some quality GL4 lube I'm going to swap that in to the tranny so I don't damage the bronze synchro rings.

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EDIT: On further research, according to the ALLdata entry for my truck, factory spec is for GL4 or GL5 oil in the manual transmission. I went ahead & ordered a gallon of RedLine MT90 (GL4) anyway. I'll probably try swapping back & forth over a period of weeks to months and see if I notice any diference at all in shift feel, noise.

 

Came back to clear this up.
GL4 only for those poor synchros.

 

It's interesting that the factory spec says "GL4 or GL5". Also, semi-authoritative sources seem to disagree. This first vid is from a guy who specializes in manual transmission build/repair/rebuild, has looked at 1000's of transmissions and never seen bronze synchro damage related to GL5 oil use. He also took the trouble to contact transmission engineers for their opinions on the matter. He thinks the misplaced concerns often heard around the internet are related to old (>2 decade or more) GL5 formulations which have long since been amended not to cause any problem. His view seems to align w/ the factory spec of either GL4 or GL5 for the 6-speed MT.

https://www.youtube.com/watch?v=Z7NpsMZ9pBQ&t=724s

This other guy is more of a generalized mechanic, not a transmission specialist, & he insists that it is important to use GL4 specifically:

https://www.youtube.com/watch?v=3le9ZT5HUlk

I think the first guy probably knows what he's talking about since all he does is work on manual transmissions. The second guy may be knowledgeable across a lot of areas but not immune to occasionally picking up & repeating out-of-date info.

I got a gallon of Redline MT90 GL4; for the minimal cost & trouble of changing out fresh Lucas GL5 for a well-tested Redline GL4, compared to even the remote risk of possible long-term accelerated synchro wear, GL4 is the conservative, safest choice.

 

TLDR: In my 2014 V6 TRD off-road 6-spd manual, Redline MT90

$30 75W-90 Synthetic Gear Lube - 1 Quart (Pack of 2)

75W-90 GL4 drives, shifts, and sounds noticeably better than Lucas 75W-90 GL5.

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This was a complete apples-to-apples comparison, as follows:

- Drained & filled MT about a week ago with new Lucas 75W-90 GL5 synthetic.
- Drove it on 2 test drives, separated by about 4 days, on the same route of about 12-14 miles, 50/50 city / highway, flat to low slopes, very aggressive shifting & accel (after warm-up), hitting high highway speeds (>85 mph) for 5-10 seconds.
- Lucas 75W-90 GL5 shifted & ran acceptably but with a noticeable transmission gear whine at higher tranny RPMs.

Today, after the second Lucas test drive (to heat up the gear oil a bit,) I drained out the Lucas to a slow drip (~1 drip every 5 or 6 seconds,) then plugged it & refilled with new Redline MT90 75W-90 GL4.

Spent an hour or so cleaning up (I like to flush out my gear oil pump with water + detergent, dry it with some rubbing alcohol inside, etc.,) then went out for the exact same test drive, including aggressive accel, fast highway speeds, etc.

- The Redline MT90 was noticeably smoother immediately on pulling out of the driveway. Almost no tranny gearbox noise at all. Butter-smooth shifts.
After warm-up & aggressive accel. & shifting, there was a little bit of gear noise & the shifts weren't quite so well "padded," as one would expect from hotter, thinner gear oil. But on the whole a distinctly better feel & noise signature from the tranny.

For interest here's a photo of very lightly used (30 miles!) Lucas 75W-90 vs brand new Lucas 75W-90. I'm saving the used stuff to put in the diff of my old beater 1999 2WD Taco. This was in identical clean containers, filled to the same level, under uniform indoor lighting. Interesting to see a noticeable color change after just 30 miles. Maybe this was due to detergent package (?) in the oil dissolving some old tranny gunk. Probably also due to slight dilution with the previous old oil, which had 60K miles on it. (Not sure but I'd guess at least a few ounces of old oil stay in after a complete drain due to surface coating & interior case nooks & crannies.)