The M840T is one of the most reliable engines McLaren has made — but its oil change is the one job on the 720S where a well-meaning owner can genuinely hurt the car. There is no dipstick. The oil lives in two places that people confuse. And the "level check" is an electronic reading that lies to you if you do it at the wrong time. I've done this change several times on my car, and this is the complete procedure as I run it, with the factory details pulled from the service manual.
The M840T is a twin-turbo V8 with a dry-sump lubrication system. In a conventional car, oil drains back into a pan under the engine, a pump sucks it from the pan, and a dipstick in the sump is your level check. None of that maps cleanly onto this car.
In a dry-sump setup, the pan under the block holds very little oil. A pump moves what drains into it quickly into a proper oil tank, and the pump draws from there. On the 720S that tank sits in the rear of the passenger cell, separate from the engine itself. Two consequences matter for the oil change:
Get the refill amount wrong and you're either under-filling (the dry-sump pump cavitates and you lose oil pressure under load) or over-filling (oil foams, the pressure relief valve works overtime, and you'll see it in the pressure readings or as a whiff of burning oil under hard braking when geysered oil hits the exhaust). Both are avoidable. Here's the job.
Run the car until it's at normal operating temperature, then switch it off. Hot oil drains thin and fast, and the system is fully circulated at operating temperature. The electronic level check also behaves better when the system is at the temperature it reads at. Give it a few minutes after shut-down so the flow through the system settles — you don't want to be working under a sump that's still sloshing.
The drain plug is on the underside of the engine. Put the pan under it, back out the plug by hand once the threads break free, and let it run. It should be a clean, steady stream that tapers off. Check what comes out: color, clarity, and anything metallic. Fine metal particles at this point in the oil's life are one conversation; bright new glitter or chunky debris is another, and it's worth a scope before the next job you're planning.
The filter is the cartridge on the passenger side of the engine. Remove the old one, wipe the seat clean, and check the O-ring area. Fit the new cartridge with a fresh seal (the filter should come with one — if yours doesn't, that's the day to get one). Hand it on straight, then snug it per the procedure in the manual. Cross-threading a cartridge filter is annoying; starting it square is the whole trick.
Fit the new crush washer to the sump plug, torque it to 40 Nm, and refill through the fill point with your 8.5 liters of 5W-40 ACEA A3/B4. Pour slowly toward the end — you're not filling to a hard maximum, you're getting close, and the electronic check is what tells you where you actually are.
Here's the part that trips people up. The instrument cluster reads the oil level, but the reading is only trustworthy when the system is in the state the ECU expects it to be in for a measurement. If you read the cluster immediately after refilling, with oil sloshing around the tank, the number you see is not your level.
Follow the level-check procedure in the manual — it exists for exactly this reason, and it walks you through getting the system to a stable, readable state. In practice: the car on level ground, system cycled the way the procedure says, then read the cluster. If the reading is low, top off a little and re-check. Top off a little, re-check. Don't guess a liter in and walk away.
Once the level is correct, run the engine briefly, confirm no leaks at the plug or filter, check the cluster for any oil-pressure warnings, and then reset the service interval in the instrument cluster so the next change is scheduled properly. Take a photo of the drained oil for your records if you're logging maintenance — it's the single cheapest wear indicator this car has.
McLaren's interval is 12 months or 10,000 miles, whichever comes first. I run mine by the calendar even when the miles are low — oil degrades with time and heat cycles, and this engine runs hot. If you track the car or drive it hard, I treat the oil like a consumable I check at every fuel stop rather than every month, because the consumption rate goes up with temperature and the dry-sump system doesn't forgive a slow leak the way a wet sump does.
If your consumption suddenly jumps — more than a healthy amount between two normal checks — the first thing I investigate is the PCV system. A stuck PCV valve builds crankcase pressure that pushes oil past the rings, and it's a far cheaper fix than the engine work you'd be diagnosing if you ignored it.
The oil change is the easy job on this car. The ones that scare people more are the RCC II hydraulic flush and the EPB service mode for caliper work — both written up in the same style, with the factory cross-references. For the full procedure text, torque tables, and part references, open the service manual and go to the Powertrain section.
Have a different result from this procedure, or a step you think I got wrong? Tell me — I'd rather fix the page than have the next person follow a bad step.