Forte Diff & Gear Treatment: every question customers actually ask

Forte Diff & Gear Treatment is a £20 gear oil additive for a differential, transfer box or manual gearbox that whines, hums or howls, or a manual box that has gone stiff and notchy because the gear oil has degraded or the gears and bearings have started to wear. It goes into the existing EP gear oil without draining and is recommended for limited-slip diffs; it is not listed for automatic gearboxes, and it will not silence a bearing or gear that has already failed.

By Phil Clements, BMW Tool RentalUpdated 16 September 2026

What is it and what does it fix?

A supplementary additive package that puts the protection back into tired gear oil.

Diff & Gear Treatment is a fully balanced supplementary gear oil additive package for gearboxes, differentials and transfer boxes. Gear oil is asked to survive extreme pressure between gear teeth and high temperatures, and as it ages its additives shear down and stop protecting. Forte's treatment improves the chemical and shear stability of the oil so it keeps lubricating and protecting under heavy load and heat, which is where a tired diff starts to sing.

The symptoms it is aimed at are whining, howling or humming from the differential or transfer box, and stiff or notchy gear changes in a manual gearbox, when those are caused by degraded oil, bearing wear or gear tooth wear. It is a fluid treatment, so no fault code is associated with it.

Forte list four benefits: it quietens excessive gearbox and differential noise, gives smoother changes in a manual box, reduces gear friction and operating temperature, and reduces gear tooth wear to extend the life of the unit. It is specifically recommended for limited-slip differentials, where it can help with clutch chatter and wheel hop. Reviews and the order button are on the Diff & Gear Treatment product page. The Forte Seal Conditioner page covers the companion product and when to add it.

How do I use it?

No draining. It is designed to work through the oil already in the unit. It is often used alongside Forte Seal Conditioner; the Forte Seal Conditioner page explains when that pairing makes sense.

Identify which unit is making the noiseA rear diff whines with road speed and changes on and off the throttle; a gearbox noise changes with gear selection and clutch. Each unit has its own oil fill, so treat the one that is complaining.
Check the unit uses EP gear oilThe treatment is compatible with all gear oils formulated to international manufacturer specifications for extreme pressure (EP) gear oil, which covers manual gearboxes, front and rear differentials and all-wheel-drive transfer boxes.
Add the treatment through the filler plugStraight into the existing gear oil. Do not drain first; if the unit is at its full mark, let out enough to make room so it is not overfilled.
Drive normallyThe treatment mixes and works as the unit warms and the gears churn the oil. Judge the noise and the shift feel over the following days and weeks of normal driving.
If the unit is also weeping, add Seal ConditionerFor a manual gearbox on EP gear oil with a fluid leak, Forte's guide recommends using this treatment alongside Forte Seal Conditioner, which conditions the hardened rubber seal.

When will it not help?

It restores the oil. It does not rebuild the gears. Keep the £20 in these cases.

Automatic gearboxes. Forte list this product for manual gearboxes, differentials and transfer boxes. Automatics are not on that list and run on ATF, not EP gear oil; for harsh or slipping automatic changes use Forte Automatic Transmission Treatment.
A bearing or gear that has already failed. The treatment reduces friction, heat and tooth wear and quietens noise from degraded oil and early wear. A collapsed bearing, chipped tooth or a diff with metal in the oil is past what any additive can do.
Noise that is not the diff or gearbox. Wheel bearings, tyres and propshaft joints all produce whines and hums that get blamed on the diff. If the noise changes when you swerve or corner rather than with the throttle, check the wheel bearings first.
A unit that is low on oil. Check the level at the filler plug before anything else. A diff that has run low whines for that reason, and an additive does not replace missing oil.

Forte Diff & Gear Treatment

The gear oil additive for a whining diff, a humming transfer box or a notchy manual gearbox. Improves the shear stability of the existing EP gear oil, cuts friction and heat, and is recommended for limited-slip differentials. Add it without draining. Ships worldwide from the UK; UK delivery free on orders of £35 or more, postage and any duty shown at checkout for the USA, Canada, Australia, New Zealand and Ireland.

£20Free UK delivery on £35+ orders

Order Diff & Gear Treatment Full product page

Forte Seal Conditioner

For a manual gearbox, diff or transfer box that is also weeping oil. Conditions hardened rubber seals so they swell back and grip; Forte recommend it alongside Diff & Gear Treatment where an EP gear oil unit has a leak. Not for automatic gearboxes or wet-belt engines. Ships worldwide from the UK; UK delivery free on orders of £35 or more, postage and any duty shown at checkout for the USA, Canada, Australia, New Zealand and Ireland.

£18Free UK delivery on £35+ orders

Order Seal Conditioner Full product page

Diff & Gear Treatment questions, in the order people ask them

My rear differential whines on acceleration. Is there an additive to quiet a noisy differential?

Yes, if the whine comes from degraded oil or early bearing and gear tooth wear. Forte Diff & Gear Treatment improves the shear stability of the gear oil so it protects properly under load, and quietening excessive differential noise is its first listed benefit. A bearing that has already failed will not go quiet with any additive.

What causes a differential to whine or hum?

Most often the gear oil has degraded, the level has dropped, or the crown wheel and pinion or their bearings have started to wear. Whine that rises with road speed and changes on and off the throttle is the diff; a hum that changes when you swerve is more likely a wheel bearing.

My manual gearbox is stiff and notchy, especially when cold. Will this help?

Stiff or notchy manual changes caused by degraded oil are on Forte's list for this product, and smoother gear changes in a manual transmission is a stated benefit. If the box is notchy because of a worn synchro or a clutch that is not fully releasing, that is mechanical and the treatment will not change it.

Is it a friction modifier for a limited-slip diff?

It is a fully balanced additive package that Forte specifically recommend for limited-slip differentials, and it can help with the clutch chatter and wheel hop an LSD produces when its oil is tired. Check your LSD's oil specification first; the treatment is compatible with EP gear oils formulated to international manufacturer specifications.

Does Forte Diff and Gear Treatment actually work?

It does what a gear oil additive can: restores the chemical and shear stability of tired oil so friction, temperature and tooth wear drop and noise from that cause quietens. It does not rebuild worn parts. Customer reviews are on the product page.

How long does it take to quieten a noisy diff?

It starts working as soon as the unit warms and churns the oil, so judge it over the following days and weeks of normal driving rather than the first trip. If there is no change at all after a few weeks, the noise is not coming from the oil.

How do I add it? Do I have to drain the diff?

No draining. Remove the filler plug and add the treatment to the existing gear oil, then refit the plug. It is designed to work through the fluid already in the unit. Step by step on the Diff & Gear Treatment page.

How much do I need for a diff, and can one bottle do a gearbox and a diff?

Forte's instruction is simply to add the treatment to the unit's existing oil. The gearbox and the diff are separate oil fills, so each unit that is complaining needs its own treatment. Message Phil with the car if you are unsure which unit to treat.

Should I change the gear oil or just add the treatment?

If the oil is old, black or has never been changed, change it and add the treatment to the fresh oil so it starts from a clean base. If the oil is in reasonable condition, the treatment goes straight in without draining.

Will it fix a leaking gearbox or diff seal?

Not on its own. For a manual gearbox on EP gear oil with a fluid leak, Forte's guide recommends using Diff & Gear Treatment alongside Forte Seal Conditioner, which conditions rubber seals that have hardened and shrunk. A split or torn seal needs replacing.

Can I use it in an automatic gearbox?

No. Forte list it for manual gearboxes, front and rear differentials and all-wheel-drive transfer boxes, all of which run EP gear oil. An automatic runs ATF and has its own product, Forte Automatic Transmission Treatment; see the harsh automatic gear changes page.

Is it compatible with my gear oil?

It is fully compatible with all gear oils formulated to meet international manufacturer specifications for extreme pressure (EP) gear oil, which is what manual gearboxes, diffs and transfer boxes are filled with. If your unit specifies a non-EP fluid, message Phil before ordering.

Can I use it in a transfer box on a 4x4?

Yes. All-wheel-drive transfer boxes are one of the three applications Forte list, alongside manual gearboxes and front and rear differentials.

Does it work in a BMW rear diff?

Yes, in the same way as any rear differential on EP gear oil: it restores the oil's protection, cuts friction and heat and quietens whine from degraded oil or early wear. Check the level first, because a low diff whines for that reason alone.

Will it reduce the operating temperature of the diff?

Reducing gear friction and decreasing operating temperature is one of Forte's four listed benefits. Lower friction and heat are also why it reduces gear tooth wear and extends the life of the unit.

Does it set or clear any fault codes?

No. It is a fluid treatment with no OBD fault code association. A gearbox warning on the dash needs diagnosing on its own.

Can I use it in a motorbike gearbox?

The applications Forte list are car manual gearboxes, differentials and transfer boxes. A motorbike with a shared engine and gearbox oil is not on that list, so message Phil with the bike before ordering rather than guessing.

Is a noisy diff safe to drive on?

A light whine from tired oil generally is, and the treatment is the cheap first step. A whine that turns into a grinding, clunking or a sudden loud howl, or metal in the oil, means stop and get it looked at before it lets go.

How much is Forte Diff and Gear Treatment?

£20 with free UK delivery on orders of £35 or more from BMW Tool Rental's Diff & Gear Treatment page. With Seal Conditioner for a weeping unit it is £38 for both.

Do you deliver outside the UK?

Yes. UK delivery is free on orders of £35 or more. Orders to the USA, Canada, Australia, New Zealand, Ireland and the rest of Europe are sent tracked from the UK, with postage and any duty shown at checkout before you pay. There is a dedicated page for US customers.