A car on LPG: do you need a special “gas” oil, and when should you change it?

Gas is still nothing exotic here: by the count of the consulting group A-95, in 2025 autogas made up a fifth (21%) of all the fuel sold at Ukraine’s filling stations. And every owner of a car with an LPG conversion kit hears two opposite pieces of advice. Some say that on gas the oil does not turn black, so it can be changed less often. Others say that gas “burns hotter”, so the oil has to be changed twice as often, and only a special “gas” oil at that. In fact both pieces of advice miss the mark, and, as always, what decides is the carmaker’s requirement.

What changes in the engine when it runs on gas

What gets better

Petrol enters the cylinder as a liquid, and while the walls are cold, some of it runs down past the rings into the sump and thins the oil. Gas does not do that: in an ordinary LPG kit the vaporiser (a reducer heated by the engine coolant) turns the liquefied gas into vapour, and vapour has nothing to wash the oil off the walls with.

But an engine with an LPG kit starts on petrol, and the controller switches it over to gas only once the vaporiser has warmed up — in winter that takes noticeably longer. Cars with factory-fitted LPG work the same way: Renault and Dacia models with the ECO-G system always start on petrol. So a car on gas still spends the cold start and the first part of the warm-up — the hardest minutes for the oil — on petrol.

And one more advantage: soot. Gas burns very cleanly, there is almost no soot, so on gas the oil stays light-coloured for a long time.

What gets worse

It is commonly believed that an engine runs hotter on gas. As far as the oil is concerned, this has not been proven. The exhaust valves, though, really can run hotter: the gas mixture burns more slowly, and if the ignition timing has not been adjusted for gas, the mixture is still burning during the exhaust stroke.

The chemistry is not clear-cut either. In large stationary gas engines, as one of the biggest makers of industrial oils writes, the oil really does age not only through oxidation but also through nitration: nitrogen oxides from the combustion chamber react with the oil => nitro compounds form => the oil thickens => sludge and varnish appear. But in small high-speed gas engines, by the same maker’s data, the oil is aged mainly by oxidation — just as in petrol engines. So whether the oil in a car running on gas ages faster than on petrol is an open question, and we will come back to it. Blow-by past worn rings, meanwhile, speeds up oil ageing on any fuel.

The real weak spot: valves and seats

Gaseous fuel leaves almost no protective oxide film on the valve face and the seat, so metal seizes onto metal and wears quickly. For gas engines, engineers have even developed dedicated valves with hard-faced seating surfaces and seats made of powder metal. And researchers at the Cracow University of Technology found burnt valves and seats in converted engines without hydraulic tappets.

A valve that has burnt out on gas is a matter of materials, clearances and the LPG kit’s tuning, not of the oil brand. If your engine has no hydraulic tappets, have the clearances checked more often — that is a job for a mechanic, not for a can of oil. As for the valve fluid that LPG fitters offer, it is fed into the intake together with the gas, that is, on the fuel side rather than through the oil; whether your engine needs it is a question for a mechanic, not for the oil.

Do you need a special “gas” oil?

Neither API nor ACEA has any gas categories: in API the S classes are for spark-ignition engines and the C and F classes for diesels, and for cars ACEA has the A/B and C classes. As far as the oil is concerned, a car on gas is a petrol engine, full stop.

Take the technical data sheet of a “gas” oil. One well-known European maker has a 5W-30 sold “specially” for engines running on gas: ACEA C3, mid-ash (0.70% sulphated ash), HTHS 3.5 mPa·s — an ordinary C3 oil. The same maker’s “gas” 5W-40 is likewise ACEA C3 and BMW Longlife-04 level. The rest is promises of protection “against valve wear” without a single figure, and on the change interval the sheet honestly refers you to the carmaker’s requirements. It is like a spade labelled “for the allotment”: the spade is an ordinary one, and the label is for the buyer.

So a “gas” oil is no sin, as long as it has exactly the approval, class and viscosity that the maker of your engine requires. But it is no obligation either. As always, start with the carmaker’s recommendations. Industrial “gas engine oils”, meanwhile, are made for stationary gas engines — gas-engine power plants, compressors — and have no place in a car.

What carmakers that fit LPG at the factory say

Dacia presented its ECO-G engine as designed for gas from the outset, with a factory LPG system and the same warranty as the petrol versions. In Renault’s oil catalogue for Ukraine, the gas and bi-fuel versions sit in the same rows as the petrol ones and get the same oil: for current engines, a 5W-30 to the Renault RN17 standard. And Renault Group writes that servicing the gas version is no different from servicing the petrol one: the same intervals and the same cost.

If gas really demanded a special oil or a change interval cut in half, the first to say so would be those who give a factory warranty on a gas engine.

When to change the oil on gas

Kilometres, engine hours and severe conditions

So how much is “more often”? The studies disagree. In a Korean study from 1994, the oil from the gas car gave out fastest: the anti-wear additive was used up faster and the acid number rose. In a Romanian study from 2017, the same 10W-40 oil aged considerably less in the gas engine than in the petrol one. Both studies are small. So the “rule” of cutting the interval on gas by 30-50%, which does the rounds in the press with a nod to unnamed “experts”, is just an opinion. I have not found a single carmaker that requires this for its gas cars.

The interval in the service book is a ceiling, and on gas it is no different. After that, everything is as usual: city driving, traffic jams, short trips or taxi work are “severe conditions”, and you shorten the interval whatever the fuel (see what factors shorten the service interval).

If the car spends long stretches standing with the engine running, the odometer is fibbing: for its police cars, Ford counts every hour of idling as 33 miles (53 km) of driving. Nobody has set such a conversion for gas cars, but the conclusion is the same: many hours of idling are a reason to change the oil sooner than the odometer suggests.

Warm-up on petrol and the petrol in the tank

Remember that every cold start a car with an LPG kit makes is on petrol? On short trips, especially in winter, the engine spends a large share of its time on petrol — with fuel and condensation in the sump (more on this in the article on the oil change interval today).

One maker of LPG kits advises keeping the tank a quarter to half full of petrol and refreshing it regularly, and having the gas system checked 1,000-1,500 km after installation and then every 20,000 km or once a year (other makers have their own figures — check the manual for your kit). This is a separate item in the maintenance schedule, and an oil change does not cancel it.

A few myths about gas and oil

1. “On gas the oil does not get dirty, so it does not need changing.” On gas the oil really does stay light-coloured for a long time, but it ages all the same — you just cannot see it from the colour.

2. “A gas oil protects the valves.” What destroys the seats is not the oil but the lack of a protective film and wrong clearances.

3. “An additive in the oil will save the seats.” I will say it once more: apart from the oil, you need to pour or tip NOTHING into the engine (see oil additives: the heart of the matter).

4. “On gas the oil has to be changed twice as often.” Not on gas, but in severe conditions.

So, as far as the oil is concerned, a car on gas is the same petrol engine: put in what its maker requires and change it no less often than the service book says, and more often in the city and in a taxi. And what will help the valves is not the oil but a mechanic.