Spraying the underside of your car with used motor oil to prevent rust: A brilliant budget tip?

In the workshops of classic car enthusiasts and 4x4 drivers, a persistent tip has been going around for decades. “Why buy 'expensive' rust inhibitors? Coating your chassis with old motor oil works perfectly. It creeps into every crevice and is guaranteed to prevent rust!”

It sounds like the ultimate free solution. But although protecting your underbody with oil was perfectly normal in the 1970s, we know better now. Spraying the underside and interior of your car with used motor oil is one of the most destructive things you can do to your vehicle, your health, and your wallet.

How does this old-school method compare to a modern, bio-based system such as Fawkes Rev Prolan? Here are the facts.

Why coating your chassis with old motor oil destroys your car

The idea behind using used motor oil to prevent rust is simple: oil repels water. But used motor oil has been chemically altered by extreme heat, compression, and combustion gases. This poses major mechanical risks:

• It eats away at your suspension rubbers: motor oil is a petroleum product. As soon as it comes into contact with your suspension rubbers (control arm bushings), brake hoses, or axle boots, these rubbers will swell, soften, and eventually split completely.
• It forms a 'liquid file': Used oil remains liquid. As soon as you start driving, it attracts sand and road dust. This mixture turns into an abrasive paste that actually wears out your car's components faster.

The Prolan alternative: Prolan is 100% based on natural lanolin (wool fat). It is completely inert. This means it is 100% safe for all types of rubber, plastics, and wiring. It actually keeps rubbers supple and protects them from UV rays instead of damaging them.

Durability: How long does used motor oil really protect against rust?

If you search for spraying old oil on a chassis, you are hoping for a long-lasting result. However, the reality of its durability (how long the product actually protects) is far from ideal:

Used motor oil: 1 to 3 months

• Open chassis and wheel arches: after the first heavy rain shower or drive on a wet highway, the thin film of oil is largely washed away. In the wheel arches, the protection often disappears within a few weeks.
• Chassis rails: Because the oil remains liquid, it slowly drips out of the car due to gravity. After a maximum of 3 months, the active protection is gone.

Prolan Multi-Grade: up to 5+ years (and topped up in 15 minutes)

• Open chassis and underbody (Prolan 300/350): Prolan dries into a tough, dry wax layer. This layer is resistant to mechanical friction and pressure washers up to 170 bar. This provides a durability of 2 to 3 years, and many years longer when topped up, even in salty winters. And even longer when the car is only used in summer.
• Closed hollow spaces (Prolan 170/270/300): In chassis rails and sills, there is no mechanical wear because the Prolan becomes a dry wax. Here, Prolan 170 continues to provide active protection for 5 years or more without drying out.
• Durability easily extended: You do not need to treat your car completely again. Simply topping up the areas that appear to need it is enough. For this, we have Top-up kits.

The hidden dangers: Health and the environment

Deliberately using old oil on your chassis is no longer just a handy trick nowadays; it is an economic offence.

1. Carcinogenic (IARC Group 1)

While being used in the engine, toxic substances (PAHs) and heavy metals accumulate in the oil.

According to the International Agency for Research on Cancer (IARC), part of the World Health Organization (WHO), used mineral motor oil is officially classified as IARC Group 1: Carcinogenic to humans. Spraying this oil means you are directly inhaling these carcinogenic particles.

2. Huge environmental fines

Because the oil quickly washes off your car, it ends up directly in the environment.

The Human Environment and Transport Inspectorate (ILT) classifies used oil as hazardous waste. Discharging it on public roads or into the soil is strictly punishable under the Environmental Management Act. After all, one litre of used oil can contaminate a million litres of clean groundwater.

Quick overview: Used motor oil vs. Prolan

Feature	Used Motor Oil	Fawkes Rev Prolan
Durability on open underbody	1 to 3 months (washes away quickly)	up to 3+ years (resistant to 170 bar). Easily extended
Durability in hollow spaces	Maximum 3 months (drips out)	5+ years of active protection. Easily extended
Effect on rubbers	Disastrous (rubbers swell and split)	100% safe (keeps rubber supple and protected)
Health risk	Extremely high (Proven carcinogenic)	Zero (natural and non-toxic)
Legal status	Illegal for open use / discharge	100% environmentally friendly and legal

Conclusion: Leave the garage-find myth in the past

Do you want to protect the underbody of your car with oil? The savings from a free leftover of used motor oil are an illusion. The costs you will incur later to replace damaged suspension rubbers, fail an MOT inspection, or the risks to your own health are simply not worth it.

With the multi-grade system of Fawkes Rev Prolan, you mimic the only good property of oil (its extreme creeping ability), but with protection that lasts for years and is completely safe for both people and machines.

Choose quality that really stays put. Discover the full Fawkes Rev Prolan rust protection range and give your car the safe protection it deserves.