The 8 Best Transmission Additives

Low-speed shudder, the shake some automatics make when the torque converter locks up, is the one target of the Lubegard Instant Shudder Fixx. You squeeze the whole small tube into the dipstick tube or fill port, with no draining, and the shudder often fades within a short drive. It isn't for CVTs, dual-clutch gearboxes or older Fords that use Type F fluid. It won't cure slipping from worn clutches, and it's harmful if swallowed and irritates skin and eyes.

Older automatics that slip, hesitate or shift hard are the target of the Lucas Transmission Fix, a thick, non-solvent additive. It's also meant to stop most seal leaks and help the fluid run cooler. It suits automatics and light-duty manuals, but not CVTs or dual-clutch units. It pours slowly, so warm the engine and use a long funnel. Shifts often smooth out, but results on leaks vary, and it can't repair burned clutches or electrical faults.

CVTs, the automatics that use a steel belt or chain instead of fixed gears, need their own fluid, and the Lubegard CVT Recharge & Protect is made for them. The dose depends on how much fluid your system holds, and one bottle treats up to 10 quarts. Lubegard says full effect can take up to 150 miles and recommends a fluid and filter change alongside it. It's a supplement, not a cure for a worn belt or pulley.

Gum and varnish that can make the valves inside an automatic stick are what Hot Shot's Shift Restore is meant to dissolve. Erratic shifts, hesitation, rough downshifts, overheating and noise are its targets. One bottle treats about 20 quarts of fluid, enough for most cars and many pickups, and it's labeled safe for CVTs. It replaces an equal amount of fluid, so drain some first. It costs more up front, and only the label backs these claims.

High-mileage automatics get a preventive boost from the Lubegard 60902 ATF Protectant, the company's original red formula, instead of a fix for one symptom. Its wax-based chemistry is meant to restore friction and anti-wear additives in tired fluid, and the carton says it conditions seals without swelling agents. Only Lubegard's own wear test backs it. A single bottle suits most cars. It isn't for CVTs or Ford Type F fluid, and it won't correct a mechanical failure.

Sea Foam Trans Tune is a solvent cleaner meant to loosen the sticky residue and varnish behind rough or erratic shifts, and it also works in power steering and hydraulic systems. The whole can goes into the filler, and the seller says it's safe with regular and synthetic fluids. It's for automatics, not manuals, and makes no claim to stop leaks or slipping. It's flammable and harmful or fatal if swallowed, so store it away from heat and children.

A seal conditioner for slow leaks, the ATP AT-205 Re-Seal targets the kind that leave a few drops under the car, not a puddle. It's meant to soften hardened rubber seals without swelling them, in engines, transmissions, power steering and more. Keep it out of brake systems. Drips often slow for months, though results depend on the leak. It won't fix a torn seal, a cracked pan or a loose bolt, so find the source first.

BG ATC Plus is an automatic transmission conditioner from a company that sells mostly through repair shops. It's meant to condition seals, resist the oxidation that darkens and thickens old fluid, and help control shudder, in new or used fluid. Add it at a fluid change. The can and seller give few numbers, and the label says to read the safety data sheet first. It's pricey per ounce and isn't labeled for CVTs.