Manual five speed trans. oil

[werewolf]

Ok, i have a 1999 4x4 3.0 manual five speed. Cold it shifts nice. When it warms up it still shifts ok but not as smooth. I been to the tech articles and seen that Red line is a oil of choice. But is is saying 75w and there another that says 5w to 40w. the oil in my trans. looks and fills like atf. So i know everyone has a different, thought on oils & additives. This truck has 180,00 got it used don't have any back ground on it. Won't to change the trans. oil flush it and go with new oil and maybe a additive. I'm in west texas don't have much cold or snow here. But we have crap load of hot weather. So let me get some of you guys thoughts on this. thanks

 

[RonD]

1990 and up Rangers used M5OD-R1 manual transmissions, they use ATF, yes automatic transmission fluid, Ford specs Mercon V

You can use a synthetic ATF many like it, some find the Mercon V better.

I would look at both options and the YEAR of the transmission with the users comments on Mercon V or synthetic, I believe synchros were changed but not sure of specific year.
And Mercon V changed as well but any Mercon V made after around 2003 will be fine in newer and older M5OD-R1s

 

[AllanD]

ACTUALLY Mercon-V is "synthetic"...

Generally the M5OD-R1 shifts best on Regular Mercon, but the later transmissions
(your 1999 is definatly "later") seem to work well with Mercon-V

The plastic shifter "springs" don't like the EP additive in most gear oils,
but as my personal transmission does not have those "springs" (I run a Hurst)
I've been running my transmission on Valvoline 70W-85 Synthetic gear oil for years...

 

[Shran]

ATF or engine oil. Nothing else.

Having owned a variety of trucks with M5OD's... some of them are better with engine oil, some are better with ATF. My '96 Ranger shifted a lot better with engine oil but had a nasty grinding issue going into 2nd and 3rd with ATF, 5w-30 cured that. My 92 Explorer seems to prefer ATF. '88 Ranger seems to be alright with engine oil. It's cheap enough to experiment and see what works best.

 

[4x4junkie]

I've seen hard-shifting issues develop after putting Mercon V into the M5OD, I would suggest to use what it calls for: Mercon.

(2008 & up M5ODs call for Mercon V. Anything earlier specs Mercon)

Due to some licensing shenanigans, bottles of Mercon ATF are now generally labeled "Dex/Merc", but it's made to the same specs as Mercon by well-established manufacturers such as Castrol and Valvoline (so it is still available).