Use the oils that the manufacturer recommends and change them at a frequency commensurate with your use.

 

Absolutely. I'm just wondering if any additives also work or if that's some complete waste of money.

 

With any car. running gear lubrication additives are a no, no.
If the car is properly serviced at intervals or sooner, then there should be no issues with fluids or their respective connected mechanics, normal wear and tear excluded.

Petrol, fill up once with premium fuel every 6 months or so. (pre-winter, late spring to take advantage of outside temperatures)
Diesels, the only fuel additive worthwhile are dpf cleaners as a precautionary measure or when issues arise.

The remainder is Snake Oil, as fluids are already balanced to manufacturers specifications.
Adding 3rd party additives may degrade the foundation chemical compound balance hence reducing the lifecycle of the main fluid. no matter what is claimed on the advert or label.

 

Thank you very much!
I'm far from the UK but I'm in this forum for the abundance of knowledge. Here in Israel premium petrol is non-existant and honestly I am certain that even the Octane rating is inflated (95 is ubiquitous, 98 could be found if you look for it) and temperatures are quite a bit hotter. Fuel system cleaning additives seem to be recommended here by mechanics and others, however I try to service my car every 7-10,000 KMs (even though Dacia recommends 15) to maintain good working order.

Thank you for the knowledge; I appreciate it!

 

I've used (petrol) Red-X in all my cars, both with carburetors and injection. I have never had blocked jets or injectors, nor had a fuel pump fail. I once added a gear box and engine additive that claimed to reduce wear. Again no problems.

 

Thank you very much!
Red-x isn't very common around me but I understand that it is mostly snake oil and not all the buzz, nor will it replace good maintenance.
All the best!