Not a good idea at all i would highly recommend don't do it
However yes one can mix same brand oils with same specifications
that is what i do for generator, my elemax generator oil capacity is 0.6L
i use shell 20w50 which is 0.7L so always some oil remains in bottle
every time during oil change some oil remains so i keep them in one saparate shell bottle
aur after few oil changes ek 0.6L ki bottle tyaar hojaate hai (H)