Blind Man Morris
Morris Minor Engine Oil
Why Morris Minor Engine Oil Turns Black — and What 20W-50 Really Means
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If you’ve ever changed the oil in your Morris Minor, taken it for a short drive, and found it black already — don’t panic!
It’s not the oil breaking down — it’s doing exactly what it’s meant to do.
Why the Oil Goes Black
Your engine oil doesn’t just lubricate. It also cleans, cools, and seals. Every explosion inside the cylinders produces tiny bits of carbon, soot, and metal.
Older engines like the Morris Minor’s A-series let a small amount of combustion gas slip past the piston rings — called blow-by — and the oil picks up all that soot. That’s what darkens it.
If your oil is turning black quickly, it means it’s holding the dirt in suspension instead of letting it stick inside the engine — exactly what it should do.
🧴 Why Quality Oil Matters
A proper classic 20W-50 oil such as Duckhams, Castrol, Millers etc has:
Zinc (ZDDP) for wear protection,
Detergents to keep the engine clean,
Corrosion inhibitors to stop internal rust.
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And coats the metal surfaces and bearings with a thin layer of oil, which doesn't drop to the bottom like budget oils.
Cheaper modern oils often lack these additives or are too thin for older engines, leading to low pressure and faster wear.
What 20W-50 Actually Means
Those numbers come from the Society of Automotive Engineers (SAE) — the people who standardised oil ratings.
The “20W” means it behaves like a 20-weight oil when cold (“W” stands for Winter).
The “50” means it behaves like a thicker 50-weight oil when hot.
So 20W-50 oil flows well enough for cold starts but stays thick enough to protect the bearings once hot — perfect for classic engines with larger tolerances.
🧪 How They Measure It – Centistokes Explained
Oil thickness is measured in centistokes (cSt), which shows how easily the oil flows.
In a laboratory, technicians use a glass viscometer — basically, a thin glass tube — and time how long it takes the oil to flow through at 100°C.
The slower it flows, the higher the centistoke number, and the thicker the oil.
For reference:
Water = 1 cSt
Classic 20W-50 oil = about 18 cSt @ 100°C
Syrup = 100+ cSt
That flow rate determines which SAE grade the oil fits into:
SAE Grade Viscosity @100°C (cSt)
SAE 30 9.3 – 12.5
SAE 40 12.5 – 16.3
SAE 50 16.3 – 21.9
So when your tin says 20W-50, that “50” means the oil measures roughly 18 cSt at 100°C, which is ideal for a Morris Minor.
When To Worry
Dark oil is fine.
But milky oil means coolant, gritty oil means wear, and burnt-smelling oil means overheating.
If it’s smooth and black — that’s perfectly healthy.
🧠Blind Man Morris Tip
> “Black oil isn’t bad oil — it’s busy oil.
Change it regularly, use a decent 20W-50 with zinc, and give your Morris a good long run to keep it clean inside.”
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