Do you know what's IN it?
I recall many moons ago, folks who owned Porsches, bad things happened when you mixed bases......like, never add Castrol to Valvoline, and vice-versa.
If you don't know, don't add, just change it (and filter).
Most old Jags we use 20W-50, nowadays seems to be Castrol of some type.
My 64 still has a tag on the fill cap....."Premium 20W-40", which I've not seen for a long time.
Think 20W-50, especially if it's well broke in....