Correct usage of lbs. as in "pounds" of weight

In scientific publications, units of measurement are almost never pluralized when abbreviated. This should remain true for general use, as well. You should therefore never write "lbs." You should technically not need a period after "lb" either, unless it's at the end of a sentence.

The abbreviation "lb" comes from the Latin libra, which is short for libra pondo, or "pound weight." And in any case, the plural of libra would be librae, not libras.

And, again in scientific papers, there is always a space between the quantity and the unit.

Grammar Girl has a related article, “Units of Measure.” The key points:

• Put a space between numbers and units of measure. Typographers prefer a thin space, but you shouldn't generally need to worry about it.
• Abbreviations are generally the same for singular and plural units: While lbs isn't incorrect, lb is better for formal and scientific use. Never pluralize metric system units like meters and grams.
• Period usage varies with style guide. They're more common in American writing. However, never use periods with metric system units.

I suggest writing 2.3 kg instead of any of 5 lbs., 5lbs., 5 lb. or 5lb. Historically, however, the forms “5 lb” and “5 lbs” appear to have been used more than either of the others (according to ngrams for 5 lbs,5lbs,5lb,5 lb,5 lb . [where 5 lb . represents 5 lb. ie has a period after lb as part of the search]) :

Note, if you click on the book links on the ngrams page, you will note that in many instances the 5 is after a decimal point, eg, “12.5 lbs.”.

Note, naturally 5 lb occurs more often than other forms because 5 lb occurs in every instance of any of {5 lbs, 5 lb, 5 lb., 5 lbs.}. Ngrams for 5 lbs,5 lb,5 lb .,5 lbs . shows that 5 lb. and 5 lbs. nowadays occur with nearly equal frequency, the latter slightly more frequently. Before 1980, 5 lb. occurred rather more frequently than 5 lbs.

"lbs" is never correct, lb is an abbreviation for Libra (Latin for Balance/Scales), and is both singular and plural it should always be lb.