Just an addition to JMW's advise. If you don't know the mass, use a smaller container, e,g a tin can,
-measure its weight/ mass, and dimensions. Pour the liquid-mix to half or full to the brim, measure its weight, -deduct from can's mass= liquid mass in can.
-divide container volume to can's volume per mass of liquid= total mass of liquid.
- density of water (known), density of type of oil(you can find from perry's handbook), the rest is maths.
Now I got a question for all in the Petro-world, I 'm a Chemi looking to work for either consulting or refinery, (my experience is 14 years in Semi-conductors, design, manufacture etc- same instrumentation, piping, except we use mostly vacuum, oil industry uses positive pressure. Any suggestions, pointers, recommendations, etc, etc.