Doc's Additive + Fluids

Doc's Diesel Oils, Additives + Fluids

Protection in Every Bottle.

Filters and pads get the glory, but fluids do the quiet work of keeping a diesel alive. The right oil protects your bottom end under load, the right transmission fluid keeps shifts crisp at 200,000 miles, and the right fuel treatment makes up for what today's ultra-low sulfur diesel took away. Doc's Diesel Oils, Additives + Fluids is the one collection that covers your truck from the fuel tank to the crankcase—premium fluids and targeted treatments formulated for hard-working Cummins, Powerstroke, Duramax, and EcoDiesel engines. Everything here has one job: keep your rig running strong between services, not just during them.

From the Fuel Tank to the Crankcase—Covered.

Start with the foundation: Doc's Full Synthetic Heavy-Duty Diesel Oil, engineered for wear protection in severe conditions, and our ATF+4 Synthetic Transmission Fluid for smooth, consistent shifting. Then protect the fuel side—our 24/7 Diesel Treatment + Cetane Boost goes in every tank to restore the lubricity ULSD lacks, while Interval Diesel Treatment delivers a concentrated fuel system cleanout every 5,000 miles. Running an emissions-equipped truck? Our DEF fluid and DEF Guard additive keep your SCR system clean and derate-free. And when winter hits hard, SOS Cold Weather Rescue reliquefies gelled fuel down to -25°F. From daily protection to emergency fixes, if your diesel drinks it, we've got the right formula—including money-saving bundles like the 1-2 Punch and Workhorse Kit that pair treatments into complete protection strategies.

Formulated to OEM Standards. Zero Snake Oil.

The additive aisle is full of miracle claims. Doc's isn't that. Our fluids are formulated to meet or exceed OEM specifications and manufacturer warranty requirements, our treatments are third-party tested where it counts, and every bottle comes with straightforward dosing instructions—how much, how often, and what it actually does. No mystery formulas, no vague promises. Just proven chemistry, clearly labeled, from a company that runs diesels too. No guesswork. No BS.

Doc's Makes It Easy to Treat Your Truck Right.

Not sure which oil your engine calls for, or how much? Use our Vehicle Selection Tool—enter your year, make, and model, and we'll show you the right fluids and capacities for your setup, from a 12-quart 6.7L Cummins to a 7-quart 3.0L Duramax. Every product page also spells out the dosing and treatment schedule, so you'll know exactly what to add at every fill-up, every oil change, and every season change.

Free Shipping on All U.S. Orders $99+

Fluids and additives are the easiest products in the store to stock up on—they don't need fitment checks twice, and you'll always use them. Grab your oil, a season of fuel treatment, and a jug of DEF in one order, and every order over $99 ships free anywhere in the U.S. Your truck takes care of you. This is how you return the favor.