My mother-in-law borrowed my car, and in her politeness / gratitude went so far as refill it with petrol.

The problem is, the car takes diesel! Thankfully there was no lasting damage, but even in my limited knowledge I know that this is not good for a car.

So if I'm loaning my car to someone, particularly if they're not going far, I don't expect them to replace the fuel they used, but I'd still like them to know what fuel it takes in case they decide off their own bat to do so.

But I can't help thinking the mere mention of a fuel type will be taken as a hint that I do expect them to.

For e.g. even if I were to say, "I'm not saying I expect you to refuel, but if you do decide to, it takes diesel, not petrol" - it practically obliges them to refuel.

Furthermore I'd like to avoid telling them not to refuel at all - if they decide to on their own, they're welcome to! I very much appreciate the thought.

  • Aren't there any labels near the gas cap that state which kind of fuel your car uses? Is the transmission manual or automatic?
    – Tinkeringbell
    Jul 21, 2022 at 13:16
  • 6
    @Tinkeringbell - manual. The label is probably not obvious enough for someone who doesn't go out of their way to check (maybe that's the solution - to stick a big obvious label on there)
    – komodosp
    Jul 21, 2022 at 13:42
  • 1
    I drove a borrowed white van once. Tank was empty, I checked the manual. Guess what it said: "If your car has a petrol engine, fill the tank with petrol. If your car has a Diesel engine, fill the tank with Diesel". That's no joke. (Just realise you meant manual vs automatic, I thought you said "check the instruction manual").
    – gnasher729
    Aug 22, 2022 at 9:08
  • As an aside - to me it is totally normal and expected to check the type of fuel a car needs before refueling. As a driver you should know that there are different fuels, and most cars actually have a sticker next to the fuel intake. If you suspect someone might put in the wrong fuel instead of checking, I just would not give them the car.
    – sleske
    Sep 1, 2022 at 8:36

4 Answers 4


Try saying

The tank has fuel, but in case you run out, it's a diesel car.

To make it seem more innocuous, you could be more specific with how much fuel the car already has, for example:

I've already filled up the tank, but in case you need to refill it, it takes diesel.

While saying it, also try to emphasize on "in case", so they know you don't expect them to fill it up unless they need more fuel.

I have a scooter that I use in college, and my friends borrow it on occasion. Scooters can only take Petrol, so I often need to remind some of my friends who don't have scooters of their own to not fill Diesel by mistake. So I have used this/similar lines to this multiple times to convey a similar sentiment.

  • 1
    If you could add a line about how things worked out? Do you think your friends feel obliged to fill up? Is your scooter often returned to you, filled up? Adding personal experience doesn't mean just saying that you did this, but describing the results and how well they align (or differ, sometimes) from the goal presented in the question
    – Tinkeringbell
    Jul 21, 2022 at 19:46

When I'm in this situation, I package the needed information with an extremely improbable hypothetical situation where the information will be necessary to them. So for example, I might say something like,

If you end up deciding to travel to Japan and need more fuel, you'll need to get diesel.

That way, the information is delivered in a 'this is to help you' package, but they can use the information to help them refuel your car if they decide to.

  • 1
    I'd say this is a good way, at least as good as the most-voted one.
    – Frank
    Sep 12, 2022 at 20:44

The simplest way to do it is to get a mention of the type of fuel in there without talking about filling it up:

It's got a full tank of diesel, so don't worry about running out

Or, if you don't have a full tank:

The tank is three-quarters of diesel, which should be good for XX miles.


Instead of needing to tell someone which might feel pushy, you could also just put a bright 'DIESEL' sticker on your fuel tank cap*. While this isn't a IPS kind of answer, I did want to provide it as an extra solution, a fallback.

Apart from the sticker, a simple "In case you need to fuel, it's a diesel" will (in my experience) cover 99% of the situation. Most people I've told this register this as a 'I'm being careful with my car'. By using the 'in case' part you're not telling them anything, just telling them a "gotcha", something they might miss.

*I'm not native english, I mean the (often black) plug you need to unscrew, not the outside hinge one.

  • If I properly understand, this is not warning the borrower unless he's opening the tank to refill. And OP wants to communicate ahead of that. Can you please clarify ?
    – OldPadawan
    Sep 11, 2022 at 16:55
  • @OldPadawan correct. I'm aware this is IPS, but I think this is a valuable 'plan B', in case you forget to tell someone (like OPs experience). I've rephrazed the question a bit, better like this?
    – Martijn
    Sep 12, 2022 at 8:00

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