[Roundsworth]

Hi everybody! I know this is a subject that has been covered in depth, but, I have a couple of concerns with the GL 4 rated oils I have found. They are labeled for outboard motor lower units and come in two varieties, full synthetic and synthetic blend. Would either of these oils be satisfactory in my 2006 5 speed transmission? Cheers!

[Arizoni]

IMO, either of the GL4 oils you mention should work fine.
You might want to avoid the GL5 oils though.

Some of the earlier made GL5 oils are reputed to cause problems with brass parts.
Although the oil companies have fixed this problem, some of the older produced oil might still be on the shelf.
Also, the GL5 oil is needed for limited slip hypoid gear systems but your transmission does not have any hypoid gears so this oil isn't needed.

As for semi-synthetic and full synthetic gear oil, I don't see any great advantage to either one of them.

Synthetic oils have much better temperature stability and are less likely to break down due to extremely high temperatures.  The temperatures in your gearbox will never approach these temperatures so IMO, even the old dino- oils will work fine.

[Roundsworth]

The GL 5 being bad for bronze bushings frightened me a bit. The transmission has recently produced GL 5 oil in it right now. I read some disturbing things about it and wanted to replace it ASAP.

[Bilgemaster]

The GL 5 being bad for bronze bushings frightened me a bit. The transmission has recently produced GL 5 oil in it right now. I read some disturbing things about it and wanted to replace it ASAP.

That's probably a good idea. If you've got Amazon Prime, you can get a quart of this Mercury Quicksilver SAE 90 straight GL-4 on sale right now for "Primesters" for just $9.97 with free shipping. Normally it's $20 to $25 in marine supplies joints. It's just what the doctor ordered and user manual specifies for your gearbox.

You know, I just sort of signed up by mistake for Amazon Prime while getting something for my kid one day in a rush, and about a year later I'm really glad I did. I'm sure it's saved us some cash, and it turns out their streaming media service is not entirely woeful. Hell, the first season of The Marvelous Mrs. Maisel alone is worth the price of admission.

[mattsz]

Mercury Quicksilver SAE 90 straight GL-4

what a quaintly redundant name...

Hell, the first season of The Marvelous Mrs. Maisel alone is worth the price of admission.

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