BMW Repair: 1999 323i oil changes, mile intervals, quality oil

Question
I just bought a 1999 323i, and I want to change the oil. The car was run with synthetic for the first 80k miles, but the third owner did not use synthetic for the last 10k miles. Should I switch back to synthetic? Also, do you recommend I keep using 91 octane, or does 87 octane suffice?

Thanks,

Ryan

Answer
I have been under the assumption that once you use synthetic oil you should stay with it.But I am more of an old school type of guy and use a good quality oil such as Castrol.I have not used the synthetic but do change the oil at 3000 mile intervals.Checking the owners manual would give you the exact fuel octane to use.But I think that 87 works well and its 20 cents or more cheaper