Q: I have a GMC Sierra 2500 HD Duramax 4x4 with a six-speed standard gearbox. I bought it new in 2001 and now have 98 000 miles on it. Over the years I always made sure the maintenance was done as per the owner manual, but something bugs me. Nowhere in the schedule maintenance does it talk about gearbox oil change. They cover the engine oil, the engine coolant system, and all kinds of minor items, but they never talk about the standard transmission, the transfer case, the rear and front differentials, and what about the power steering and the hydraulic clutch? How many years can we go without replacing the oil in all these components? I understand they use synthetic oil these days, but are these synthetic oils good for the life of the truck?
A: I scanned through several related model-year GMC pickup service schedules, specifically the Duramax Diesel owner's manual supplements, and I see where you're coming from. According to General Motors there's no need to change the six-speed manual transmission fluid or the transfer case fluid (unless you have an automatic), and you should periodically check, top-off, but never replace the front or rear axle fluids. Engineers believe that under "normal operation" the synthetic lubricants will hold up indefinitely, or at least as long as the predicted lifespan of the truck.
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The advanced lubricity and thermal resistance of synthetics outweigh that of petroleum based products, and the manual transmission, transfer case and axles are sealed (or nearly sealed) components that limit or eliminate incoming contaminates -- such as normal blow-by gasses into engine oil. Drivetrain technology also has lightened the load on lubricants by lessening internal friction. As far as brake fluid, clutch fluid, or power steering fluid replacement, nowadays they're not typically part of a manufacturer's maintenance schedule. With all that said, if it's my truck and I want to keep it for a long time, I would try not to let any drivetrain fluid go far beyond the 100,000 mile mark. Of course, this is just an opinion. If you agree, go to GMC service and be sure to use the correct replacement fluids. There may be updates since your owner's manual went to print. Dealership service department's maintenance schedules also often disagree with that of the manufacturer.