Neither of the cited sources in the wiki article say that 75% of the RN's fuel oil comes from Mexico.This Wikipedia article also states the claim and offers two sources if you want to dig further
https://en.m.wikipedia.org/wiki/Mexico_in_World_War_I
In addition:
In 1917, the Royal Navy consumed 12,500 tons a month, but had a supply of 30,000 tons a month from the Anglo-Persian Oil Company, using their oil wells in Persia.
Which is supported buy the source cited (The History of the British Petroleum Company: Volume 1). Given the investment in APOC by the UK specifically to supply fuel oil fro the RN, such a dependence on Mexico is highly unlikely. The same book also mentioned British pre-war reluctance to use Mexican sources specifically because of political unreliability.