No topic causes more garage debates than 2-stroke oil. Too little oil in the fuel and you risk a seized top end; too much and you foul plugs and load the power valve with carbon. The right answer depends on which generation of KTM 300 EXC you own — because between carburettor, TPI and TBI models, the way oil gets into the engine changed completely. This guide covers all three, plus the gearbox oil that riders forget about until the clutch starts complaining.
Know Your Generation First
- Carburettor models (up to 2017): classic premix — you mix 2-stroke oil into the fuel yourself.
- TPI models (2018–2023): transfer port injection with a separate oil tank and pump. No premix — the bike meters oil automatically.
- TBI models (2024 and newer): throttle body injection for fuel, but the oil pump is gone. These are premix engines again — a detail that has caught out more than one owner upgrading from a TPI bike.
If you are unsure which system your bike has, check your model year in our owner manuals library before touching the fuel can.
Premix Ratio for Carburettor and TBI Models
KTM specifies a 1:60 ratio with a high-quality fully synthetic 2-stroke oil for its premix engines. In practice, riders run anywhere between 1:60 and 1:40 depending on load, terrain and jetting — but if you change the ratio, the carburetted engines need jetting to match. Richer oil ratios (more oil) effectively lean out the air–fuel mixture.
Quick reference for mixing:
- 1:60 — 16.7 ml of oil per litre of fuel (KTM's specification)
- 1:50 — 20 ml per litre
- 1:40 — 25 ml per litre (common for sustained high-load racing, requires jetting)
Mixing Tips
- Mix in a dedicated fuel can, never in the bike's tank — pour the oil first, then the fuel, and shake thoroughly.
- Use fresh, high-octane fuel. Premix ages: don't store mixed fuel for months.
- Measure with a proper mixing cup, not by eye. A "splash" is not a ratio.
TPI Models: Feed the Oil Tank, Not the Fuel
TPI bikes (2018–2023) inject oil from a separate reservoir under the seat. Your jobs are simple but non-negotiable:
- Never add oil to the fuel — the injection system already meters it, and premixing on top doubles the oil going through the engine.
- Top up the oil tank regularly — as a habit, check it at every second or third fuel stop. The dashboard warning light is a last resort, not a maintenance plan.
- Use the same quality of full-synthetic 2T oil you would use for premix. The pump does not make cheap oil safe.
Which 2-Stroke Oil to Use
Use a fully synthetic 2-stroke oil designed for high-performance off-road engines — an oil that burns clean, protects at high RPM and keeps the power valve free of carbon. A proven option we stock is Silkolene Comp 2 Plus (1L), an ester-based full synthetic. You'll find more options in our oils and lubricants collection.
Whatever brand you choose: stay consistent. Switching oils constantly makes jetting and plug reading guesswork.
Gearbox Oil: The Forgotten Service
The 300 EXC's gearbox and clutch share an oil bath that is completely separate from the premix. KTM specifies a 15W/50 gearbox oil (Motorex Top Speed 4T is the factory fill) with a capacity of roughly 0.8 litres — always confirm the exact capacity and interval for your model year in the owner manual.
- Use a JASO MA2 rated oil — the wet clutch depends on it. Car engine oils with friction modifiers will make the clutch slip.
- Change it often — the gearbox oil volume is small and works hard. Hard enduro riders change it every few ride days; a common rule of thumb is every 10–15 engine hours.
- Check the magnetic drain plug — fine grey paste is normal wear; glitter or metal flakes are a warning sign. If you're seeing debris alongside clutch fade, read our clutch replacement guide.
Quick Change Procedure
- Warm the engine for a few minutes so the oil drains fully.
- Remove the drain plug, drain, clean the magnet, refit with a new sealing washer and torque to the manual's specification.
- Fill with the specified quantity, check the level per the manual procedure, and inspect for leaks after the first ride.
Symptoms of Getting It Wrong
- Constant plug fouling, excessive smoke, spooge from the exhaust: too much oil for your jetting, low-quality oil, or too much low-RPM lugging.
- Detonation, overheating, scored piston: too little oil or stale premix — stop and investigate before it becomes a rebuild. If it already is one, our full engine rebuild guide walks through the job.
- Clutch slip after oil change: wrong gearbox oil spec — drain it and refill with a JASO MA2 oil.
FAQ
Can I use the same oil for premix and the TPI oil tank?
Yes — a quality full-synthetic 2T oil serves both. The difference is where it goes, not what it is.
My 2024+ TBI bike has fuel injection — surely I don't need to premix?
You do. TBI injects fuel, not oil. There is no oil pump on the 2024+ generation: every tank must be premixed, per KTM at 1:60.
Is more oil safer?
Only up to a point, and only with matching jetting on carburetted bikes. Follow the manual unless you have a specific, tuned reason not to.