[gvdlans]

Ben,

You say you can only get estimates, but this is not the case. Gasoil is not a pure component but a mixture of hydrocarbons derived from crude oil (e.g. by atmospheric destillation, boiling range of gasoil is 250-371 °C). The exact composition (and therefore the exact density) depends a.o. on the source of crude oil. Other remark is that you always have to define the temperature if you ask for a density, since density changes when temperature changes.

Some examples, all at 15°C:

Gasoil derived from an Algerian crude: 835 kg/m3
Gasoil derived from an Arabian crude: 857 kg/m3
Gasoil derived from a Venuzuelan crude: 910 kg/m3

So if you don't have a specific gasoil in mind, the correct answer is to give a range such as 800-910 kg/m3.

[Radionise]

I agree with gvdlans,

I'd suggest you to follow the info given by gvdlans for your assignment.

You can do a few things in selecting an absolute value for the density,

1. taking the average between the lower and upper limit that you found from your literature.
2. selecting one of the values quoted by gvdlans above.

What ever you do, it is important to state your assumptions and reasons for the assumptions in your report. Those are what lecturers normally interested; how you justify your selection.
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my TIPS for all students:

Calculations are always not a problem in chemical engineering, but finding the appropriate data to 'fill in the blanks' are always the biggest challenge. And that is also how it would be in the industry later.