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Old 10-01-2012, 01:02 AM   #1
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changing rear end oil to synthetic oil

this is to the diesel pushers. my question is has anyone changed rear end oil to synthetic and recorded if any fuel milage change? i know putting 140 weight in a f150 (my bosses axle seal went out) from synthetic to get it home (truck was under waranty) killed the fuel milage and i think that was a dana 60. so lets hear the feed back.

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Old 10-01-2012, 02:34 AM   #2
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We run 85-140 mobil one in semi trucks and in my jeep wrangler. That the weight you need in the DP also. Go ahead and change it out, it can't hurt anything. This is a multiple vis oil and will be like a 85 weight when it's cold, and like a 140 weight when it's hot. Best of both worlds! Valvoline also makes a good full synthetic 85-140 gear lube.

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Old 10-01-2012, 06:34 AM   #3
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Doubt you will find a mileage increase, but you will find a temp decrease.
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Old 10-01-2012, 11:50 AM   #4
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Didn't see any increase in mileage. Don't know about a temp drop as I have never measured it.
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Old 10-01-2012, 07:44 PM   #5
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Changed to synthetic in the differential years ago, no change in mileage... On the other hand, would be suspicious of possible damage to the rear end if there was a noticeable drop in fuel mileage after loosing oil from a seal failure... any noise or vibration??
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Old 10-02-2012, 07:34 PM   #6
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I do know that syn gear lube does not properly lube below 100 degrees F. I have rebuilt MANY rear diff, in pass cars and trucks that have used syn gear lube from the manufacturer. This wear was caused from the vehcle only going short dist, and the lube never warming up. As for a vehicle that is on the hwy, syn lube is great. lowers diff temp, and increases rear gear life.
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Old 10-02-2012, 08:24 PM   #7
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You got me.. why would synth not lube properly below 100F?
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Old 10-04-2012, 05:46 PM   #8
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Originally Posted by Midniteoyl View Post
You got me.. why would synth not lube properly below 100F?
Me too. When syn first came out, I personally put a dyno and syn bottle in the freezer to see which one was more liquid and the syn won, its viscosity remained stable it did not jell.. Similarly using a tobe thrown frying pan ran the test they showed on TV again the syn was more stable where as the dyno was like water.
Not too sure about the seals not liking it, but from my understanding most synthetics have additives in there to assist in keeping the seals from degrading.
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Old 10-04-2012, 06:17 PM   #9
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Changed to Synthetic no increase in mileage, but now do not have to change the lube as fotten per the manufacturer. I like less maintenance costs.

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Old 10-04-2012, 07:20 PM   #10
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I changed to Mobil one 75-90 and found a 2% improvement in fuel
economy over the next 8000miles. There are many other benefits
as well.

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