Apologies if this is not appropriate for this site or too simple for the members here but I am not great with this.
If I have something measured in ml, can I convert it to grams by assuming 1g -> 1ml?
If I have something measured in oz, can I convert it to grams by assuming 1 oz -> 28.3495 grams? Or is there a difference between "ounce" and "fluid ounce"?
Having trouble telling these things apart and what conversions make sense and which do not.
Grams are mass and milliliters are volume. 1 ml of something very dense will weigh much more than 1 gram. You need to multiply by density to make the conversion and the units will tell you that. To go from length$^3$ to mass you need to multiply by something that has units $\frac {\text{mass}}{\text{length}^3}$ and the density is that.
Avoirdupois ounces are a unit of mass as are grams, so you can just use the conversion factor you quote. Fluid ounces are a unit of volume, about $29.57$ ml. There are also Troy ounces, used for things like gold, that are units of mass but different from the avoirdupois ounce.