I only joined when I was 19 or 20 so I was terrible before hand!
Main stupid thing I did before I had a clue was go nearly 25,000km on cheap dino 15w40. That's not the worst part, about 10,000 km into that fill I overheated the engine big time and drove well over half an hour with zero coolant, probably cooking that cheap oil which I then went on to drive another 15,000 km with!!!
Makes me cringe thinking about it but all it did was make my lifters rattle a bit from the build up it caused in them.
Other than that I once used a full bottle of lucas stabilizer plus 4 liters of 15w40. Must have been at least equivalent to a 60 weight and it knocked like crazy on the bottom end in a morning. Switched straight back to 10w30.
Before then on my first car (Mitsubishi Diamante with that Mits/Chrysler 2.6 engine) would burn more oil that it did fuel so I used to shove 2 or 3 bottles of cheap stop smoke (its probably more than twice the thickness of Lucas, took 10 mins to empty a bottle in!) that engine blew a head gasket at 299k, I was aiming for 300k and missed by about an hours worth of driving!