Originally Posted By: MBS500
You can add any oil pretty much( up to adequate level). Even plain old 50 weight from grandpas garage.
UMM! Modern oils mix OK, but if your Grandpa was using some weird racing oil for his model T Ford I would give it a miss!
If you are getting oil analysis done on occasions, don't mix oil types as it confuses the results and can make it harder to interpret them.