Ideally the spring should match the struts, paired, but you can just replace the springs. For me, I did the opposite -- I replaced OEM struts and shocks with Bilstein, right after having purchased a GT. It's more firmer, less bouncy.
I made posts 10W-30 and 5W-40 appear too thick, feels sluggish accerlating. 5W-30 seems just right. Someone answered my post "pacebo". I'd say you and I can feel the car better than some folks.
You're young and financially also, so just keep it stock, keep up with maintenance, is my advice. When you're older and financially comfortable, then mod all you want.
I checked my reverse lock sleeve by trying to move side to side in all directions. It does not move. Can you check yours? If it moves, then it must be missing a plastic sleeve bushing or something.
Try locktite so the knob is "bonded" to the shifter rod thread tight. This will stop the rattle. You need to use Loctite stick, part no 37684, so it won't run down like the liquid one.
1. No problem burning 87 especially on highway driving. I might get slight ping starting out from zero.
2. I bought an Ecoboost, only to sell it to get me a GT eight months later. You just can't do without the 5.0 V8 growl and beautiful exhaust tone.