My car suffered a large oil leak which lead to oil spraying down the side of the bodywork. It was't very clean when this happened, so I have a mix of oil residue with ingrained dirt over the gelcoat.

What's the best way to get rid of this before I give the car a proper wash? Any kind of wiping will surely leave it's fair share of scratches. I'm thinking an aerosol degreaser would loosen the worst of it.

I have a couple of cans of this stuff which claims to be safe for use on plastics...

https://uk.rs-online.com/web/p/precision-cleaners-degreasers/4348924/

Any other tips?

Jek

Autoglym do a tar remover that's quite effective.Linky

Once "clean" use a rubbing compound on the gel coat, it will polish it up well. Mind that gel coat when looked at very closely is covered in minute bubbles which are great at filling up with dirt so a good wax polish is always a good idea to help fill these up and keep out dirt. Even the best gel coat is still a poor second to paint but still has a lot of advantages.

Pledgetype furniture polish is great for removing oily finger prints.
For swirl removal you get get special utlra fine rubbing compounds including ones specially for plastic.

My first kit car was an abandoned 70's beachbuggy that despite being in a barn the whole floorpan had turned to dust and the body was completely covered in chicken poop so it looked quite something. With some rubbing compound and a power mop this was the shine I got out the gel coat, hard work but quite an amazing transformation