What fluid SHOULD i run in a stock 8.8?

fiveoho · Dec 4, 2007

gearheads say run synthetic.
summit tech guy says sythetic will eat the clutches. he says not to run it ( he suggested some sort of lakewood fluid ).

frition modifier: everyone i have asked says pour the full 4 oz bottle in with the fluid. the summit guys says do not use it unless the rearend starts making noise, then add enough to stop the noise. just my opinion but i just dont trust the advice of the summit tech.

i called my local ford dealership. he said his books only go back to 97 ( rearends are the same as far as i know ) and it called for a 80/90 fluid.

what do i use? im leaning toward using what originally came in the car.

go-stang5.0 · Dec 4, 2007

I dunno about the synthetic gear oil. I can tell you I have been running it for 4 years no problems. As far at the friction modifier...I ran out of gear oil just short of fill up and so I used 2 4 oz. bottles of friction modifier to top it off. Plus the gear oil supposedly had friction modifier in it too so who knows how much is actually in there. Been running like that for 2 years and have no problems. BTW I used the blue FM from ford racing...not the gold colered Motorcraft stuff you get from the dealer. I dunno if that makes a difference.

SublimeRT · Dec 4, 2007

The general rule of thumb is not to run synthetic if you're using a clutch type diff. like the stock trac lock. Like you were told it is too slick for the clutches. I've always used regular 80/90 and 1 full bottle of Motorcraft friction modifier with trac locks and never had any problems. The blue FM from Ford Racing that gostang used is probably fine, too.

fiveoho · Dec 4, 2007

so the 80/90 is correct and is what i want. is it three bottles i will need ( doesnt it take like 2 3/4 ) ?

and 1 bottle of friction modifer

HISSIN50 · Dec 4, 2007

Clutches are a wear/sacrificial item.

I use synthetic in all the rear ends. I do it because the initial flow to bearings is so far superior and it handles heat so much better.

Remember, FM lowers the coefficient of friction (makes the fluid more slick). I also dont use any unless I hear chattering while turning.

Later 8.8's called for synthetic 85/140 as I recall. That tells me they can handle synthetic and a numerically greater viscosity (though the synthetic 140 flows better than dino 90).

You cant go wrong either way. You know how the stock (dino) fluid did for you and if you were satisfied with it, no need to change. If you see extreme temps or something like that, the synthetic can be nice (even if it does wear clutches faster, which I'm not sold on when it comes to street cars).

Good luck.

gcomfx.com · Dec 4, 2007

I got a whole setup of Amsoil + their modifier for a web banner on my club site. So that's the route I went.

Always before I went with regular gear oil at Oreillys and modifier. Only reason I changed was because I bent an axle.

PUNISHER RACING · Dec 4, 2007

synthetic w/ friction modifier

The Green GT · Dec 4, 2007

I used 2.5 cans of Lucas oil and a can of FM that came with my gears.

WHITE94COBRA · Dec 4, 2007

Synthetic + FM, I have it in all of my cars. It's been in the 94 for years, gears and clutches always look great when I change the fluids. I change the tranny and rear end fluids once a year, right before the new year. I'll snap some pics if anyone wants to see.

Mustang5L5 · Dec 4, 2007

The 99+ Mustangs use synthetic fluid as OEM fill in the 8.8's.

There is a tag on the rear that reads SYNTHETIC FLUID ONLY

I just changed my fluid at 50K and put mobil 1 syn back in. Unsure if there was a clutch material change between the 99+ and the older cars, but i always thought they were the same.

GTA_V6_Mustang · Dec 4, 2007

I use wynns full synthetic 85-140 and their fiction modifier with zero issues so far on my 166k plus rear end.

fiveoho · Dec 4, 2007

several different opinions on this. lots of you say synthetic. i suppose if i run synthetic or the 80/90 gear oil ford calls for, i will be ok either way provided i use friction modifier with each?

heard that certain fluid(s) will not mix well with the friction modifier. cant remember what it was..... any truth to that?

SublimeRT · Dec 5, 2007

I suggested the regular 80W-90 with the modifier because that's what my Ford manual for my car says, and like I said I've had good luck with it. Looking at all the replies though it almost seems like it doesn't matter.

I've heard something about not using modifier with a certain fluid, but I can't remember which. If I had to guess I would say not to use it with synthetic since it is already a little more slippery. But again with some people saying they use FM with synthetic and some saying they don't use any at all, it seems like it doesn't matter either.

Just get whatever you want I guess.

HISSIN50 · Dec 5, 2007

There are certain LSD's (aftermarket) that don't like synthetics. I've not heard of certain gear oils not mixing with FM (though extra FM might not be needed).

Putting synthetics in as a factory fill is expensive. If it's not needed to make it through the warranty period, I dont know why it would be done. We 'upgrade' the fluids all the time.

When Ford went to synthetic in the 8.8, the weight increased to 85W-140 IIRC (same number I listed in my first post - I havent looked it up to know for sure. Maybe Mike will remember). I feel someone saw a need for greater viscosity but needed the synthetic properties to make it work (cold 85-140 dino oil can take forever to flow, relatively speaking).

That said, what Blaise and a few others said (about using an OEM weight dino fluid) is valid because it's worked this long.

I never saw any risk or harm in switching to a synthetic so I did/do. I use it on every pumpkin oil change (on all the cars).

fiveoho · Dec 5, 2007

the main reason im so concerned about getting the correct fluid is because i just ordered a new trak-lok from summit. i have been having issues. heres the story:

when cruising ,usually 60-70 and i let off, there is some vibration. its not terrible but is def noticeable. another was i described it was that it felt like slack somewhere in the driveline. we put the car up on stands to make sure the DS bolts were tight. what he found was a INCH of play in the DS where it connects to the rearend. i could spin the DS back and forth about a inch. the rearend has about 6-8000 miles on new FRPP 4.10s, timken bearings and new seals. and axle seals too i think. when i got the car back last year from the guy ( different guy that looked at last night ) who set up the gears, they were whisper quiet. i havent had any track time, i may have spun the tires twice since there were in. i drive the car like a old man. i just dont see this being driver error. what could be wrong here?

if i replace the factory trak-lok with a new one, i hope this will solve the problem. i just hope the ring/pinion are not hurt, like i said they only have about 6-8000 miles on them.

whats my chances of NOT having to replace the ring/pinion again and do all new bearings/seals again?

WHITE94COBRA · Dec 5, 2007

You may have a carrier bearing or planetary problem. That can cause a lot of slop. I'm willing to bet if he set up your gears correctly this is where your gettin the slop from.

fiveoho · Dec 5, 2007

WHITE94COBRA said:
You may have a carrier bearing or planetary problem. That can cause a lot of slop. I'm willing to bet if he set up your gears correctly this is where your gettin the slop from.

would a whole new trak-lok solve this problem if that is the cause?

95LiterGT · Dec 5, 2007

fiveoho said:
when cruising ,usually 60-70 and i let off, there is some vibration. its not terrible but is def noticeable. another was i described it was that it felt like slack somewhere in the driveline. we put the car up on stands to make sure the DS bolts were tight. what he found was a INCH of play in the DS where it connects to the rearend. i could spin the DS back and forth about a inch. the rearend has about 6-8000 miles on new FRPP 4.10s, timken bearings and new seals. and axle seals too i think. when i got the car back last year from the guy ( different guy that looked at last night ) who set up the gears, they were whisper quiet. i havent had any track time, i may have spun the tires twice since there were in. i drive the car like a old man. i just dont see this being driver error. what could be wrong here? bearings/seals again?

Either your pinion gear has loosened up or you have too much backlash between your carrier (ring gear) and pinion gear. If your pinion flange (what drive shaft bolts to) is moving front to back as you turn the driveshaft, your pinion gear has loosend up.
Or if when you rotate the driveshaft the pinion flange does not move front to back and you have an inch of rotational movement on the driveshaft, then it has too much backlash.
This doesn't sound like a trak-lock issue to me. Good luck.

fiveoho · Dec 5, 2007

maybe its not the trak-lok

the fluid:

i have asked a ton of people and i have just as many different answers. i called the dealership. he first said 80/90. then he said ford now says to use 75/140 ( which is $18 qt there ). friend of mine who runs the local NAPA told me to use lucas 85/140 for heavy duty trucks. other use synthetics. other say dont use it. i dunno what to get. i would go with what ford told me but they just sounded uncertain on the phone.

Mustang5L5 · Dec 5, 2007

HISSIN50 said:
When Ford went to synthetic in the 8.8, the weight increased to 85W-140 IIRC (same number I listed in my first post - I havent looked it up to know for sure. Maybe Mike will remember). I feel someone saw a need for greater viscosity but needed the synthetic properties to make it work (cold 85-140 dino oil can take forever to flow, relatively speaking).

75W-140

I went with Mobil 1 and the bottle says friction mod is already added...so i never added any

What fluid SHOULD i run in a stock 8.8?

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