What comes out of the pump today
Swiss super 95 is as a rule E5: up to five per cent ethanol. Super plus (98) is often the calmer choice for older engines, because the knock resistance is higher and the ethanol share is often lower, or at least not higher. E10, up to ten per cent ethanol, is not the everyday fuel in Switzerland, but you meet it in neighbouring countries.
Ethanol binds water, attacks some historic seals and floats, and ages in the tank. That is why standing still and filling up abroad are two separate risks.
Carburettor and injection
A carburettor does not forgive a poorly seated seal: leaking float needles, gummed jets, soft floats. After the winter the first job is often cleaning, not "setting it up". Injection systems (K-Jetronic and related systems) suffer gummed pressure accumulators and leaking fuel distributors. Those too are damage from standing, not from driving.
If you store the car: tank full (less air, less condensate), a last warm run before the break, and in spring you use up the first tankful on purpose. The detail of the break is under Storage.
There are additives against ageing. They replace neither fresh fuel nor tight lines. For very long breaks, an empty tank plus preservation is a workshop decision, not a faith in home remedies.
Lead
Leaded petrol no longer exists in open sale. For engines with unhardened valve seats, manufacturers and specialists offer a lead substitute. Over a long period of observation the ADAC reports no proven engine damage from the lack of lead alone; if you want to be quite sure, have hardened seats fitted at the next cylinder-head overhaul and keep the valve clearance tighter.
Highly compressed sports engines and very early heads are the group where it is worth asking a workshop that knows the marque. Not every four-cylinder of the 1970s.
Diesel
Old diesel grows microbes and waxes in the cold. Standing time plus a half-full tank is the unhappy combination. Check the filter and the water separator before the season. Biodiesel shares (B7) are usual in Swiss diesel; very old injection pumps take to it unevenly.
No experiments at the test
Changes that affect the exhaust (a different carburettor, aftertreatment removed, open manifolds) belong to the entry, not to a "better sound". At the MFK, what counts is what is written in the vehicle registration certificate.