I could spend hours putting together a comprehensive list, but just a few minutes here for things off the top of my head:

Parts you can cheap out on:

1. Spark plugs - Go for what the OEM recommends. A cheap set of platinums if you car had them from the factory, or for some older cars, copper plugs work best. The oddball plugs out there don’t really do much on a modern, unmodified car. Regular plugs can be as cheap as $3 a pop, whereas some fancy plugs are upwards of $15. On a car with the correct minimum number of cylinders (8) that’s $24 vs $120 for a set of plugs.

2. Oil - As others have mentioned, cheaper oils that are API certified can be fine. I find a happy balance in cheaper off-brand synthetics. But spend wisely on the filter.

3. Additives - Be it oil, coolant, or otherwise, most additives are junk/snake oil type stuff. Save your money. Exceptions of course for modified cars or old cars that may need “supplements”

4. Octane booster - can put in the above - but making seperate. Most “octane boosters” they sell at the parts stores, if you read the label say “boosts octane 4 points!” But 4 points is only part of the equation for obtaining the true octane rating. 4 points is misleading and really means about .1 octane... Exceptions in stuff like Race Gas octane booster.

Things not to cheapen out on:

I see alot of people trying to buy the cheapest parts they possibly can, and sometimes it’s a bit scary. This list would be a lot bigger if I took the time, but means things like:

Ball joints
Tie Rods
etc. etc.