6-SPEED TRANSMISSIONS & SYNTHETIC OIL

I have a couple of questions about the Duramax and the Allison. I’ve heard this term, “double overdrive sixth gear.” What does that mean?

What is the recommendation about the use of synthetic oils? Is it OK to use synthetic? If so, does using synthetics lengthen the oil drain intervals?

I just bought a used Duramax with 31,000 miles on it. This truck had always been serviced by the dealer, but the sticker shows an oil change due in 3,000 miles. Is this correct? I am barely getting acquainted with my Duramax. I use the truck to tow occasionally and am very impressed with the performance thus far.

Vincent Blazys Via Email

Let’s begin by taking a look at the six-speed Allison 1000’s gear ratios:

First: 3.10

Second: 1.81

Third: 1.41

Fourth: 1.00

Fifth (OD): 0.71

Sixth (double OD): 0.61 Reverse: -4.49