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.

Edit: 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.

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.

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.

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.

Edit: 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.