Our engineers say 70% or 12 liters (for the classic indicators). It's just a "guideline" as I suspect, but i find it a conservative one and as the previous post say, various books go as low as 50% but I have rarely seen less than 75-80%. The best approach is to call mx when you see it approaching 70%.
On the other hand, would be nice to have an official minimum oil qty since the amplified procedures state that you should verify it before flight..oh well..boeing..
rgds.