We have a 7-yr-old Panda Eco (does a fantastic job as "the shopping
trolley").

In the recent (and ongoing) unpleasantness it was due a service, but our
garage had shut up shop. I suddenly realised that _this_ was _one_
thing I could DIY on a car these days.

Long story short: I looked at the 3 or 4 part-full oil bottles I had
had in the garage for umpteen years, and they came, together, to about a
litre less than I needed (of 10/40 oil). So I went to our (magnificent)
local motor factors and reviewed the litre bottles that they had.

Cheapest was a synthetic oil. I asked the lad there (an about-40 yrs
old lad, spent all his working life in the motor factors, and engines)
about mixing this with the oil I'd got: he said "doesn't matter - just
shove it in, as long as it's the right viscosity".

So I did. The Panda hasn't complained yet. (It does about 4000m per
year btw).

My 2p
John