Green’s Dictionary of Slang

dinkum oil n.

also dink
[dinkum adj. + oil n. (2c)]

(Aus.) the honest truth, true facts.

[Aus]Sydney Morn. Herald 2 Oct. 9/5: ‘Dinkum Oil‘ Trench Newspaper [...] No. V, vol. 1, of ‘Dinkum Oil’ war news, June 23, published in the trenches on the Gallipolli Peninsula by the Ausralians.
[Aus]Aussie (France) 4 Apr. 3/2: An’ if they wants ta argue much, you send him up ta me. / I know the real square dinkum oil. So there yer are! Compree!
[UK](con. WWI) E. Lynch Somme Mud 323: Snow [...] gives us the dinkum oil about the dinner.
[NZ]N.Z. Truth 29 Apr. 1/5: He may be putting across the dinkum oil.
[Aus]T. Wood Cobbers 117: Gold. The Dinkum Oil, as they say here. The Real Mackay.
[Aus]L. Glassop We Were the Rats 79: I’ve gotta give ya the dinkum oil.
[Aus]N. Pulliam I Travelled a Lonely Land (1957) 232/2: dinkum (dinky, dinkum oil) – true or truth.
[Aus]S. Gore Holy Smoke 42: Not even the Chaldeans – who was sorta court stargazers, like our Met Office – could give him the dinkum oil about ’em.
[NZ]B. Crump ‘Bastards I Have Met’ in Best of Barry Crump (1974) 266: It was hard at first to believe it wasn’t some kind of act he was putting on, but no, it was the dinkum oil all right.
[NZ]McGill Dict. of Kiwi Sl. 36/2: dinkum genuine or fair, often expressed as fair dinkum, dinky-di and dinkum oil (the truth).
[Aus]G. Seal Lingo 55: The terms dinkum oil or the good oil [...] are said by linguists to be a transfer of the use of oil to ensure the efficient running of a machine to the need to have good information in order to fight effectively.
[NZ]McGill Reed Dict. of N.Z. Sl. [as cit. 1988].