Yes, with exception:
you can top off with any non-synthetic without a problem, however keep in mind that the better quality "diesel-type" oils would be the best to use, not the regular pcmo (passenger car motor oil) variety which would only meet the api sn rating.
The choice should be limited to chevron delo le or shell rotella which both have a higher zinc rating of around 1200ppm. This is still a lot lower than the 306 is, but the dilution wouldn't be as bad as adding a lower additive sn oil which would only come in around 800ppm.