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Discussion Starter · #1 ·
Hello all, I just bought my first diesel yesterday; a 2009 GMC Duramax, drove home 800 miles. Very impressed with the truck but worried about prior history..The truck had low miles but has burned 5200 gallons of fuel...mileage works out to 12.4 MPG. I expect the truck did a lot of towing. Everything else is supper clean. I have changed the oil, but I want to change the trans oil. I have read a ton of info and I am wondering if I use the Allison fluid will it blend ok with whatever the prior owner may have been using, possibly Dex VI. Also it appears that low oil pressure is a normal diesel thing. I have changed the oil using Rotella synthetic 5w 40.. pressures are 28 to 30 pounds at 65mph...maybe 1700 rpms...I guess this is ok?

Anyway, this is a great site..thanks for all the info,
 

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Yes, all is normal with your truck. You can mix Transynd with Dex6, but you should do at least a double drain to adhere to the longer intervals of the Allison fluid. Oil pressure is normal and you can run plain conventional 15W40 oil with no issues.

The mileage is about right. It is about what I get in the warm months for short or "city" driving. You will get more on longer, sustained highway drives. The truck likely idled for a bit bringing the average mileage down. The Fuel Used portion of the DIC can be reset, so it may not be accurate for the life of the truck. That portion will max out at 9999 gallons.
 

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Discussion Starter · #4 ·
perfect! sounds good,, just nervous buying used...this site is calming my nerves. I'm guessing the synthetic oil also lowers my oil pressure as well, old habits die hard; Synthetic is all I ever used in my vortec max which I'am replacing with the new diesel. Love that truck but I need better towing capacity.

Thanks for the post.
 

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I'm sure you got yourself a nice truck man congratulations!! Is it all stock or are there any mods?
 

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Synthetic is fine, but you don't have to run it unless temps will be below zero. Conventional oil will outlast your change interval anyway. Synthetic will have lower initial pressures because it is thinner at lower temps, but at operating temperature the two will be the same.
 

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Welcome
 

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Congratulations! Get some pictures posted up!

I was told if you're going to change it, remove the line from the top of the transmission I believe.. blah blah blah. It is so you can get a majority of the fluid out of the torque converter to prevent contamination of the new fluid. I'm sure someone will pitch in. :thumbsup
 

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Glad to see another new owner a little nervous of what the truck may have been through. I'm in the same boat. Congratulations on your new truck.
 

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welcome friend, x2 on the pictures
 

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Welcome to the DF!
 

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Welcome to the forum:thumb
 

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Welcome.

When you change your transmission fluid, make sure you use the red oem allison filter, make sure the magnate goes back too.
 

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Welcome to the forum!
 
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