I know it has been stated in previous questions/answers that the list
of the 5% plus shareholders (as distinct from Director shareholdings)
is in the SEC filings, but where exactly is this info?  I have waded
laboriously through Annual Reports, 10-K's, Proxy statements etc, on
the SEC Website - but still cannot locate ExxonMobil's major
shareholders. 
  Can anyone tell me what form type contains this list - more
specifically, where is it?

Hello.

To answer the first part of your question, the SEC form that contains
5% ownership information is Schedule 13D.

From the SEC web site:
"Schedule 13D
This Schedule discloses beneficial ownership of certain registered
equity securities. Any person or group of persons who acquire a
beneficial ownership of more than 5% of a class of registered equity
securities of  certain issuers must file a Schedule 13D reporting such
acquisition together with certain other information within ten days
after such acquisition. Moreover, any material changes in the facts
set forth in the Schedule generally precipitates a duty to promptly
file an amendment on Schedule 13D."
http://www.sec.gov/info/edgar/forms.htm

As explained on the SEC web page, the abbreviated Schedule 13G may be
used instead of a 13D if the owner of the shares is a bank,
broker/dealer, or insurance company that has acquired the shares in
the ordinary course of business and not for the purpose of changing
control of the issuer.


Now, on to the second part of your question. I'm registered over at
FreeEdgar [ http://www.freeedgar.com ], so I ran a search for
ExxonMobil, and FreeEdgar has the following documents:
Schedule 13D (amended); filed November 30, 1999
Schedule 13D (amended); filed February 9, 1999
Schedule 13D (general); filed December 11, 1998

Unfortunately, most of the information in these filings seems to
concern changes in corporate ownership relating to the merger of Exxon
and Mobil. They might be there, so you should take a look, but I
didn't see any lists of 5% owners. However, given that a 5% stake of
ExxonMobil would be worth more than $11 billion, it's possible that
there are no 5% owners of ExxonMobil.

On a final note, I should mention that information about the major
(but less than 5%) holdings of institutional investors is found in
Form 13F filings of those institutions. Often, when you hear that some
major investor has taken a large stake in a company, that information
is gleaned from 13F filings.
From the SEC web page:
"Form 13F This is a quarterly report of equity holdings by
institutional investment managers having equity assets under
management of $100 million or more."

search strategy: sec, freeedgar

I hope this helps.

Exceptional information and a very prompt response.