My dad would put used motor oil on it

There is no such thing as "All of the sudden" stop it, stop it NOW.

Call the police immediately 

HOT SPOT ... treat with salt water and clip all the fur away from it.   It has a very specific smell that you would always recognise in the future.

Unless it spreads or gets worse there is no need for a vet. 

There are various other things you can put on it such as hibiscrub,  athlete foot powder ... do some googling as there are other things.  

If you keep hitting it with saline you should be ok... and it will scab over in a few days. 

Bring him to the vet

If there is fluid around it - take the dog to a vet first thing Monday morning!

Picture doesn't help, can't tell anything by picture.  There are so many many skin problems in dogs that it takes a Vet to examine & diagnose just what it is.  There are no Vets on this site & even if there were, they couldn't examine your dog online.

Could be a hotspot.

Could be a sore & not a rash.

Could be food allergy

Treat it as if it were a sore, clean it up & apply antibiotic salve.

Like others have said it looks like a "hot spot" that is possibly a bit infected. If it is, it's not likely to go away on it's own. I treated one on my dog during covid at home. I simply sent pictures to my vet who prescribes stuff to take care of it and an instruction list. Your best to consult the vet just in case it isn't. But until then here's what I was told.

Apply a warm damp cloth to the area 3 times a day for 10 minutes. I added the Hibiscrub provided by the vet with instructions on how much to use. I used boiled water from the stove and allowed it to cool. It's also best to use disposal able stuff but if you use a cloth as the compress it must be sterilized before using it again otherwise you're just adding the bacteria back. Allow to fully dry then add the cream given by the vet. This is a vet only cream so only a vet can give it so your dog will need to speak to a vet to get this.

Wash after walks with salt water. And get as much air to it as possible. As much as possible keep the dog from lying on that side, scratching or biting at it.

That's probably a hot spot which can happen fast, not there one say and there the next.   For now, bathe it with a diluted salt solution (or diluted Hibiscrub) and then use some medicated talc to help keep it dry.  You may need to get him seen by a vet as he may benefit from antibiotics to help stop the spread of infection.

That looks exactly like a dermatitis "hot spot".  Take him to the vet first thing Monday so they can evaluate it and prescribe a medicated powder to apply.  You might also be able to find something over-the-counter at the pet store on Sunday to get him by until you can take him to the vet.  The main thing to do for now is to keep it dry, which is what the powder is for.  And try to keep him from scratching at it.  Canine dermatitis is common in the summer.  (I'm not sure why the vets call it a hot spot; maybe just because that's a spot where dermatitis is flaring up.)