Stacey,
Changing the rear diff to synthetic is a good move, like the Allison. However, in our application the engine is not, based on the cost of the synthetics. I have the same rig as you and I use oil analysis and change my Rotella T every 12,000 miles. The oil analysis says I could get 15-18,000 on the oil, however I like to see fresh oil in there more often. As a retired HD truck factory service rep, I've had lots of experience with synthetics. There is a payback for the major fleets to use it where they run 100-200,000 miles a year, but not the way we run.
Jerry
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International 4700LP custom toter and
38' King of the Road fifth wheel
"Running the roads seeing God's Great Creation"
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