hi all i have skoda felicia 1.3 carburateur  i usally use 20w50 mineral oil that spend about 3500km to 4000 max so i think what about change to full synthetic oil that suppose it stay longer from 7000km to 10000km

so i want to know what is maximum recommended oil change for this old engines

@captain_crunch IMO you'd be wasting your money using synthetic oil in that engine.

you mean that engine wont like it to stay more than 5000km

@captain_crunch - A full explanation is long and very technical, but you won't get any of the benefits that synthetic oil offers in a more recent engine with tighter tolerances. Also there's a question as to whether or not synthetic oil will damage some of the oil seals in the old pushrod engines.

i asked that question because  mineral oils life is short from 3500km to 4000km maximum and that is a short time as it require me to change the oil filter too so i asked if synthetic oil will live longer on that engine as i found synthetic oil has viscosity 10w-60 and 10w-40 even i found full synthetic 20w-50 does it help or it will not live more than 5000km maximum

You can use semi-synthetic oil. Oil change is at 10,000 km.

does 10w-60 viscosity is ok or 10w-40