A very good question which shows that you are thinking a bit more critically than most students. Well done.
To make some sense of this stuff, you need a bit of the background (which is not addressed in the CASA training material or the usual textbooks). It just happens to be part of my stock in trade as I am a weights engineer, amongst other things.
First, have a read of CAO 100.7 at
www.legislation.gov.au/Details/F2021C01234
Section 2 Definitions has this to say on Empty Weight -
empty weight means the weight, as determined in accordance with this section of the Civil Aviation Orders, of an aircraft, including all items of fixed equipment and other equipment which is mandatory for all operations, fixed ballast,
unusable fuel and total quantities of oil, engine coolant and hydraulic fluid but excluding all other items of disposable load.
This is the current requirement imposed for Load Data sheet information. However, this definition was changed 35 - 40 years ago - I would have to dig around in the file archives to find the revision when it changed. The previous definition had a significant difference in that the engine oil, rather than being "total" oil, required "undrainable" oil.
Why the amendment ? Simple. A lot of the aircraft in earlier days were powered by big radials, for which oil was a bit of a consumable, so it made sense to start with the empty weight at, effectively, nil oil, and add in the amount to be carried for the flight as a selectable load.
Progressively, the big radials were consigned to the scrapyard or historical displays and were replaced by the usual engines we see in most piston aircraft these days. For the typical HO engine, the oil doesn't see much consumption, to speak of, so it makes more sense to have the empty weight include the oil for convenience.
Getting this changed wasn't the simple exercise you may have envisaged. A number of us who were involved with weight control got a bit fed up with having to do something with the oil when it came to Load Data Sheets and loading systems. Typically, we would schedule the empty weight per the old ANO (subsequently CAO) and then schedule another quasi-empty weight with the full oil included. This second figure would be called something different (often basic weight or whatever appealed) to distinguish it from the empty weight.
I think I was the first heretic to get sufficiently frustrated by this to short circuit the exercise and just define the empty weight as explicity including full oil. After numerous (reasonably friendly) punch-ups with the Airworthiness folks at DCA (subsequently a variety of other names and, eventually, CASA) it went quiet and, subsequently, an amendment came out with the changed definition. There is always more than one way to skin a cat, as the old saying goes.
Since then, the LDS empty weight will include full oil and references to basic weight should only be looking at role specific configuration arrangements.
When the present exams were in their infancy, the then examiner had the problem that some aircraft used undrainable oil while others used full oil in the empty weight definition. He decided that he would use both definitions in the exam context, which is why you see the mixture you have in the candidate's workbook. Does it matter all that much ? Of course not. You just need to make sure that you are riding the horse with the correct terminology and usage. A simple housekeeping matter - no more, no less.
CAVEAT If you check the entirety of 100.7 you will see that the smaller aircraft don't necessarily require periodic reweighing. This has the unfortunate consequence that you will see the old definition in various of the smaller aircraft which haven't been reweighed or otherwise had the LDS reissued for ever and a day. This, probably, is why the examiners have persisted with the dual definition setup in the workbook through until the present day.
Turning to your specific questions -
When we get a question of bravo saying Basic empty weight it means that oil will be included ..
Does it ? What does the workbook say on the matter ? If we go to page 11, note 2 says "Empty weight includes unusable fuel and undrainable oil". The examiners have long confused the issue of terminology regarding empty and basic weights but it is clear that the intent is for empty weight.
they included Oil in basic empty weight
Actually, if you check the workbook example, there is no mention of basic weight. I think this is something which has crept into the mix via Industry training material.
Could we always assume in bravo that we need to add oil
That should be the story.
Will BEW or EW wont matter
This is just a consequence of the unfortunate observation that the examiner (and Industry training) is a bit lax when it comes to terminology regarding this point. I suggest, for the exams, unless something different is clearly intended, you treat the two terms as being identical.
Specifically, though, a reference to "empty weight" should be in accordance with the 100.7 requirements. Basic weight is not defined so you will see definitions used according to whatever the WCO thought was a good idea at the time. Interestingly, 100.7 does include a rubbery definition for "operating weight" which you will see used from time to time. Basically, apart from "empty weight", any other term you come across should be checked for whatever it might mean. Strictly, the WCO should provide a definition in the LDS, as issued, for the aircraft.
By all means, post any questions you have on the subject and I'll do my best to provide a useful answer.
