Valve stem seals are small rubber seals at the top of each valve. Their job is to let a film of oil lubricate the valve stem while stopping the rest running down the valve guide into the cylinder. With age and heat the rubber hardens and shrinks, and the seal no longer grips the stem. Oil creeps past it and pools on the back of the valve.

That is why the smoke follows a pattern. Overnight the oil drains down the guides, so you get a puff of blue on the first start. Sit at lights or idle for a few minutes and oil collects again, so you get a burst of blue when you accelerate away. Once the engine is moving the pooled oil is burned off and the smoke fades. Hot oil is thinner than cold oil, which is why a car can be worse in traffic on a warm day than on a cold morning.

If the smoke is constant while driving, thickens under load, or the car uses a litre of oil every few hundred miles, the oil is more likely getting past the piston rings or being pulled through a failed crankcase breather (CCV). Those need a different fix. The point of the test below is to separate the two before you spend anything. It is often used alongside Forte Oil Fortifier; the Forte Oil Fortifier page explains when that pairing makes sense.

There are three honest answers, and the right one depends on how far the seals have gone.

• A seal conditioner in the oil (£18, two to six weeks). Forte Seal Conditioner soaks into rubber that has hardened and shrunk and restores its flexibility so it swells back to size and grips the stem again. It is not a sealant and it does not thicken the oil. Improvement is usually noticed within two to three weeks, full effect between two weeks and two months. In Phil's own customers it works in roughly 8 out of 10 cases where the seals are hardened rather than broken.
• Thicker oil film in the meantime (£18, one day). Forte Oil Fortifier replenishes the viscosity improvers in tired oil so less of it gets past any worn clearance. Forte's two-step method is to add Oil Fortifier first, drive for a day, then add the Seal Conditioner. It also helps if the engine is using oil and losing pressure.
• Replace the seals (a weekend with the right tool, or a workshop bill). The permanent fix. On a BMW N62 V8 the seals can be changed without removing the heads using the AGA tool, which Phil hires out for £220 a week. A workshop doing the same job the traditional way is a multi-thousand-pound quote because the heads come off.
• What will not work. No additive rebuilds a seal that is cracked, torn or split, and none fixes blue smoke coming from worn piston rings. If the five-minute test says rings, save your money for the mechanical repair.

The sensible order for most people is: do the test, change the breather if it is old, run the two-step additive treatment for a month, and only book the mechanical job if the smoke is still there. That is the cheapest path that does not skip a step. Dose, price and the honest limits are on the Forte Seal Conditioner product page.

Any engine with enough miles on it, but a few come up constantly in Phil's inbox. The BMW N62 V8 in the E60 545i and 550i, E63 645i and 650i, E65 745i and 750i and the E53 and E70 X5 is the classic: smoke on startup and when pulling away from lights, oil consumption climbing, no fault code. The N63 and S63 twin-turbo V8s, older M54 and M52 straight sixes, and high-mileage Vauxhall, Ford and Mercedes petrol engines follow the same pattern.

Worn seals rarely set a fault code on their own. What you sometimes see is the oil they let through fouling a plug or a catalyst, so P0300, P0301 and P0308 misfire codes and P0420 or P0430 catalyst efficiency can appear alongside the smoke. Clear the smoke and those often clear with it.

If your car is a BMW V8 and the smoke is unmistakable, read the valve stem seal tool hire page as well as the additive page. Phil has done this job himself and will tell you honestly which route makes sense for your car.