A wet belt, or timing belt in oil, is a rubber timing belt that runs inside the engine and is lubricated by the engine oil instead of running dry in its own cover. The design saves space and noise, and it is now in a lot of small engines. It also means the belt is permanently bathed in whatever the oil has become: fresh oil when the car is new, and an increasingly acidic, contaminated mixture as the service interval runs on, especially in a car doing short journeys and stop-start driving. Forte's product information for Oil System Protector puts a number on it: TBIO engines suffer up to tenfold increased stress from start-stop events.

That is the whole reason flush choice matters here. On a conventional dry-belt or chain engine the flush only has to be kind to metal, seals and the oil system. On a wet-belt engine it also has to be safe for a rubber belt that is soaking in it while it works. Forte make two flushes with the same cleaning job, and only one of them is stated as safe for that belt.

Forte New Generation Engine Flush is that one. It is an internal engine cleaning treatment added to the old oil 15 to 20 minutes before the oil change, and Forte state it as safe and recommended for TBIO engines, alongside petrol, diesel, hybrid, DPF-equipped and start-stop engines. Forte Advanced Formula Motor Flush is not suitable for TBIO engines, and Forte's own instruction is to use the New Generation product on those. Dose, price and the honest limits are on the Forte New Generation Engine Flush product page.

The two products clean the same things and are used the same way. The difference that matters is the one row that says wet belt.

If you do not have a wet-belt engine, either product does the job and the Motor Flush is £2 cheaper. If you do, or you are not sure, the New Generation Engine Flush is the safe choice and the £2 is not worth the risk. Forte describe the same benefits for both: freeing sticking piston rings, unclogging hydraulic lifters, restoring VVT operation, removing moisture, stabilising compression, and leaving a clean baseline so follow-on treatments work without sludge in the way. How to add it, how long it takes and what it will not fix are on the Forte New Generation Engine Flush page.

Forte Oil System Protector, £35. Forte describe it as particularly recommended and highly important for TBIO engines, and it is the second half of their flush-then-protect sequence: the flush cleans the old oil out, the Protector goes into the new oil immediately after the service to keep it stable for the whole interval.

What it does is the opposite of a thickener. Its additive profile is Detergents 4 out of 5, Dispersants 4, Anti-Wear 4 and Viscosity Improvers 0, so it protects and keeps the oil clean without artificially thickening it. It chemically neutralises the crankcase acids that combustion forms, which is the contamination that short journeys, stop-start driving, extended service intervals and E10 and B7 biofuels accelerate. It also prevents oil emulsification, the white sludge under the oil cap that low-mileage cars get, and Forte recommend it for start-stop, SCR (AdBlue) and DPF-equipped cars as well.

So the full wet-belt routine at each service is: New Generation Engine Flush into the old oil, 15 to 20 minutes, drain, filter, fresh oil, Oil System Protector. The Forte Oil System Protector and New Generation Flush pack has both bottles for £35 on top of the service; see the products panel below. If the engine is also using oil or dropping pressure between services, that is a different product and a different question; see the Oil System Protector vs Oil Fortifier comparison before adding anything.

The New Generation Engine Flush is the product Forte state as safe for wet-belt engines, so used as directed it is not a risk to the belt. The Motor Flush is the one not to use; if you already have a bottle of it on the shelf, put it on the family's other car.

On leaks, Forte are direct: the flush does not cause leaks. What it can do is reveal one. If sludge was temporarily plugging a seal that is already worn or cracked, cleaning the sludge out can let that seal weep. That is the seal telling you it needed attention, not the flush damaging it, and on a non-wet-belt engine it is the case where a seal conditioner earns its keep. On a wet-belt engine Forte's Seal Conditioner is excluded, so a seal that starts to weep after a flush is a replacement job.

Two habits keep it safe. Idle, do not drive, for the 15 to 20 minutes, and drain everything out afterwards with a new filter. The flush is designed to leave with the old oil; it is not meant to stay in the engine, and on a wet-belt engine in particular you want nothing in the fresh oil except the oil and the Protector.