Forte publish an additive profile for each product, scored out of five across four families: viscosity improvers, anti-wear agents, detergents and dispersants. Put the two side by side and the difference is not subtle. Oil Fortifier is weighted almost entirely to viscosity and wear: it exists to rebuild the oil film in an engine whose clearances have opened up. Oil System Protector has no viscosity improvers at all and is weighted to cleanliness and acid control: it exists to keep good oil good for the whole service interval.

The one row people miss is the viscosity line. A worn engine needs the film thickening and the Fortifier does that; the Protector deliberately does not. A wet-belt engine on a long interval needs the oil kept clean and acid-neutral and the Protector does that; the Fortifier is a restorer, not a cleaner, with detergents and dispersants at 2 out of 5. Choose by the job, not by the price. Why it suits fresh oil after a service, what it does and does not do, and the price are on the Forte Oil System Protector product page.

When the engine is telling you it is worn. Forte's list of what Oil Fortifier targets is short and specific: high oil consumption, dropping oil pressure, and blue exhaust smoke caused by oil escaping past worn piston rings and valve stem seals. If you are topping up between services, the oil light flickers at hot idle, or the car puffs blue on startup, this is the bottle.

It works by replenishing the viscosity improvers that have depleted out of tired oil. That strengthens the oil film and safely thickens the oil, which physically reduces how much gets past worn internal clearances. Its anti-wear agents form a protective film on metal surfaces to cut friction. Forte say engine noise often quietens within the first few days and the full effect takes two to four weeks.

It is also step one of Forte's two-step treatment for smoking valve stem seals: add Oil Fortifier, drive normally for one day so it mixes and slows the leak, then add Forte Seal Conditioner to restore the hardened rubber. That whole route is set out on the blue smoke page.

Dose is one full bottle into the existing oil through the filler cap, no draining, at any time. It is suitable for petrol and diesel and compatible with all crankcase oils including fully synthetic. The one hard caution Forte give is that it must not be used in vehicles or motorbikes with a wet clutch system. For best results in a maintained engine, Forte suggest adding it to fresh oil after cleaning the system with Advanced Formula Motor Flush.

Oil System Protector is the one Forte name. It is described as particularly recommended and highly important for TBIO engines, and the reason is the tenfold stress figure: a belt that lives in the oil is exposed to every start-stop event and every acid the oil accumulates. Keeping that oil clean and neutral for the whole interval, without thickening it, is exactly the Protector's profile.

Oil Fortifier's stated caution from Forte is wet clutch systems, and its product information does not list wet-belt engines either way. The product Forte explicitly exclude from TBIO engines is Seal Conditioner, along with the Advanced Formula Motor Flush. So the honest position is: on a wet-belt engine, use New Generation Engine Flush at the change and Oil System Protector in the fresh oil, both stated as suitable; for anything else going into that oil, including the Fortifier, ask Phil first with the engine code. That is the same advice the site gives on every page for wet-belt owners, and it is there because the belt is the expensive part.

Everything on this page comes from Forte's own product information for the two products. Neither is a cure for a mechanical fault: worn rings, a broken seal, a failing oil pump or a stretched belt need a spanner, and Phil will say so if you describe the symptoms to him. The Forte Oil Fortifier page covers its dose, cautions and reviews.