I went to the Mobil1 75W/90 full synthetic at 20K miles. At what interval should I change the the Mobil1?

At 40K now, tow 10K 5th wheel maybe 4K miles a year.

 

The owners manual says every 15k miles. Personally I change it once a year or about every 45k miles.

 

15K as a factory recommendation didn't suprise me after seeing the powdered milk cocktail that came out when I switched to the Mobil1. I'm asking what the the Mobil1 synthetic is good for with normal use.

 

The factory fluid is synthetic. Most, if not all, diff fluids are synthetic. I use either Mobil One or Valvoline Synthetic 75-90, and after a 45k miles, it comes out a darker shade of clear than it went in. I've never had the fluid come out looking milky though, even the first diff fluid change after it was new.

 

Yea, it really suprised me what came out.

I bought the truck new and can vouch for it has never had the pumpkins submerged in water.

Rides out good and no problems. Once a year change would put me at 20K, seems like the Mobil ought to be good for a few more miles.

 

Mike,

I use Mobil 1 75w90 also. I agree with you that the original factory fill looked like crapola at 15K when I first did the change. It looked like a gray spray paint in water.

I then changed to Mobil 1 at 15K, then at 30K....and every 20K since then. 106K currently. It always comes out looking like new gear oil. I have never had it tested. My guess is that it could easily go 50K miles or more. But I started doing a 20K mile maintenance interval on my truck.....including diffs, transmission, other stuff.........so I just stick with it.

This isn't really helping your question............I am just saying that at 20K it "looks" like new gear oil coming out of my diffs. Im gonna stick with that interval though.

..

 

My first change at about 21K looked pretty nasty but the second at about 41K wasn't anywhere as nasty looking. I went with Amsoil this time and plan to run it a lot longer, probably 50K+ and see how it looks. I tow a little now and then, not super heavy either but I still wonder on that interval and that's why I went with Amsoil.

 

Pumpkins(9"lingo) don't have to go under to aquire water....I have seen vent tubes clipped to the frame that funnel water.

 

I'm a ford guy at heart, they are all pumpkins to me!

When I drained the factory goo I poured a mason jar full and let it sit a week, no water settled out.

Not sure what they use from the factory but it looked like crap after 20K.

 

You can expect the fluid to look bad the first time or two.

I went to 85-140 synthetic in mine and it helped with the chattering some have mentioned. The last change had over 50,000 miles on it and I could not see any difference between the new oil and the old. Also the magnet was clean with only the slightest bit of gray that I had to wipe off with a clean towel to see.

Currently at about 164,000 miles.

 

I just did my diff service at 60K while running Mobil 1 75W 90 and it was just as clean as what I put in so I will be extending my drain intervals. This was their second service as I did the first service at 25K. I'll probably go 80-100K on my next change.

 

For my 2005 4X4 the Owner's Manual (page 369) says "Vehicles operated in normal service do no have regularly scheduled oil (rear & front axle) oil changes. If fluid has become contaminated with water or subjected to severe service, follow.....Maintenance Schedule B. The B schedule calls for axle fluid change every 15,000 miles.

So it depends how you use the truck. My driving doesn't require the Schedule B intervals, but I changed the axle fluid at 15,000 miles anyway to drain out any break in residue, and to take a look at how the gears are wearing in. I'll probably change again in another 30 to 45,000 miles just to take care of the truck, but it's not required by the Maintenance Schedule. It's not terribly expensive, so if in doubt, err on the side of more maintenance is my rule.

 

When I did mine the first time the oil was gray. Looking closely at the gears it looked like the gears had some type of spray on them to identify the tooth pattern from when they were built. I could wipe the white/gray film off the gears.

 

I need to do mine for the first time.Is there a gasket or are you using RTV on the cover.I may just pump it out.I have a small 12v pump that I can rig a small suction hose on.

 

The factory gasket is rubber, I just cleaned and used it again, no RTV.