Service Interval Recommendations: Redline 75W90 LS, Mobil 1 or Amsoil Univ ATF

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outrun

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Here is the reason I ask,

This is loaded but my be relevant. Redline 75W90 LS was left in differential for 68,000 miles. When diff cover removed for service, it was quite dark (but not sludgy inside). My assumption is either it was dirty from the previous iterval OR I left the the Redline in too long. The tech did hand wipe the oil residue off the valve cover .... as a test ... and the bare metal behind it was spotless. The tech who is quite experianced told me your prolly change your diff more often than most people.
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Anyway, this (darkening) would lead me to believe that the oil although changed in color had not solidified ... so it still did its job. No visible breakage was observed on the pinion and ring gears.

How many miles should the new Redline 75W90 be left in? Online reading says many things from all the way to 90k!! I use to change mine every 30-50k but that is excessive and likely waste of good oil? Then again maybe 30k would be a spot less differential and cheap insurance against over haul?

How about Mobil 1 ATF? The transmission was completely purged via a BG infuser machine and 16qts of new fluid along with a pan filter were replaced 35k ago. Since no pan service has been done since a TRANS SERVICE is prolly way over due. Do you expect Amsoil Univ ATF to have longer life?

Equipment:

4L60E Transmission (stock unmodified w/integrated in radiator cooler)

GM 10-Bolt 7.5inch Differential 3.42 w/Auburn Limited Slip
 
Synthetic Atf for me = 30K in my transmission/transaxle of the Chrysler.

Synthetic Atf for me = 50K in my Ford C-6.

Synthetic Gear oil for me = 100K.

Synthetic Gear/transaxle oil for me = 30K.

I assumed you meant rear gear cover, not valve cover. Only Oil Analysis can tell you where to limit your gear and transmission service, however 68K to 70K is a good number for gear fluid exchange in your unit. 35 K to 40 K with your transmission is in the ballpark too.

Experts here say some gear synthetics darken with use. Atf is a different story....Ruby Red is about as dark as you want to go.
 
60K on the gear lube sounds right. Many people never change the stuff so don't stress. 30K on M1 atf may be a little early. If you plan to do drain and fills on the transmission start now. If you want to flush the whole system again with a synthetic fluid wait till 50-60K and get your money out of that M1. Look at and smell the atf and judge for yourself.

I would not expect Amsoil atf to hold up any better or worse than M1. I believe these two fluids are nearly identical. I've got both swimming around in my transmission now. You may find M1 to be $1-2 cheaper if that matters.
 
For the reared, dark is nothing. If it worked for 68k and you didn't run it low, I'd go for another 68k run. I prefer 30k gear oil runs simply because some leak/vent and run low on gear oil. If you check the level and top off as needed, push it longer.
Most rearends that I've serviced were low several ounces and up to a pint of fluid, and they were hot. Top it off every 15k miles cold on a level surface and 100k on a synthetic 90wt should be easy if not overworked!

For the ATF, without additional filtration, I wouldn't push anything over 30k or 2 years. If you run an inline or bypass filter, and don't roast the tranny(adding temperature variable), there is no reason why Amsoil or Mobil1 couldn't go 100k. I won't recommend longer intervals than 30k without a good filter/magnet/cooler series spliced with the OEM ATF flow.
 
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