The name of the weight unit you choose is just a LABEL, and is altered by editing the relevant DEFINE statement in the language files, english.php .
Once you have changed that LABEL (kg to lbs, or lbs to oz), the NUMERICAL VALUE you insert into a product's WEIGHT FIELD is simply a fractionaliation of that unit of measure.
If you use the Biblical measurement system of POUNDS, you have an added difficulty of representing fractions of a pound in decimal format.
As those of us who are as old as Noah will know, 1 pound is equal to 16 ounces, so one will have to DECIMALIZE the numeric value of the ounce, so that this adds up to pound values as a shopping cart is filled.
One OUNCE (as a fraction of one POUND) in decimal notation is : 0.0625
Two ounces = (0.0625) X 2 ... etc
A product with a WEIGHT of 1lb 1oz will be represented by the value 1.0625 inserted into the product's WEIGHT FIELD, when the label you choose is "lbs".