Oils + Fluids

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Oils + Fluids Every Fluid Your Rig Runs On. Engine oil, transmission fluid, DEF, and washer fluid all do their own job — but skimp on any one of them and it shows up as wear, heat, or a fault code down the line. Doc's stocks the full lineup: full synthetic diesel engine oil in every viscosity you run, platform-specific automatic transmission fluid, high-purity DEF, and washer fluid concentrate for the gas side of the shop. Available by the bottle or by the drum for fleets and shops running through it fast. What's In This Collection Full synthetic diesel engine oil in 5W-40, 10W-30, and 15W-40, plus a 15W-40 semi-synthetic option — engineered for severe-duty towing and hauling as well as daily-driven rigs. Automatic transmission fluid matched to your platform, including ATF+4, DEX6, and ULV full synthetic formulas built specifically for Powerstroke and Duramax transmissions. Doc's high-purity DEF and the DEF System Kit to protect your SCR system. And Doc's Auto Parts Windshield Washer Super Concentrate, one bottle that mixes to six protection levels from summer bug season down to -40°F. Bulk 55-gallon drums and a 192-gallon skid are available on select engine oils and transmission fluid for fleet and commercial accounts. When To Change Them Above all recommendations, follow your vehicle's manufacturer warranty guidelines. Engine Oil — Standard Service: every 10,000–15,000 miles maximum Engine Oil — Severe Service (towing, heavy loads, extreme conditions): every 7,500–10,000 miles maximum Transmission Fluid — Normal Duty: every 60,000–100,000 miles maximum Transmission Fluid — Severe Duty: every 30,000–50,000 miles maximum DEF: top off as needed; replace at the first sign of contamination or crystallization Washer Fluid: mix to season — step up concentration as temperatures drop Built to OEM Specs. Backed by the Doc's Fitment Guarantee. No guesswork. No BS. Free shipping on qualifying orders. Stock up and keep your rig running strong.