Hi Steve, and welcome to RootsChat.
We need to be a bit careful with this because we're not allowed to post details of anyone who may still be living.
Sydney Gordon Davidson, mother's maiden surname Cruickshank, died in Aberdeen in 1980 aged 79. This matches the birth of Sydney Gordon Davidson in Fraserburgh in 1900.
The 1901 census (a transcription) lists at Rosebank, Grattan Place, Fraserburgh William Davidson, 25, fish curer; wife Mary, 28 and son Sydney G, under 1 year, all born in Fraserburgh. The index at Scotland's People shows Sydney Gordon Davidson, 10, in Aberdeen St Machar. William Gordon Cruickshank married Mary Cruickshank in Aberdeen in 1899.
Mary Jane Cruickshank, other surname Davidson, aged 81, died in Aberdeen in 1951.
Sydney Gordon Davidson married Gertrude Chesso* McDonald in Aberdeen in 1932. Gertrude Chesso* McDonald, other surname Davidson, died in Aberdeen in 1996 aged 93, mother's maiden surname Milne.
There are 22 deaths of Davidsons with mother's surname McDonald and birth date 1940 plus or minus 10 years. Of these, 7 are in Aberdeenshire. This matches a birth of Gertrude Chesso* McDonald in Fraserburgh in 1902. The index to the 1911 census shows Gertrude C McDonald, aged 8, in Fraserburgh. On the same page are Beatrice D, 7; Gowon [sic index] M, 5; and Robert W, 2. On the previous page are John McDonald, 56; Emily, 41; Edith M, 15; Dorothy, 12; Florence, 11 and Alexander, 9. In 1901 (transcription) there are John McDonald, 45; wife Emily, 32; William L, 17; Isabella G, 16; Maggie G, 15; Anne W, 14; John, 13; Edith M, 5; Dorothy, 2; Florence, 1. John McDonald married Emily Milne in Aberdeen in 1893, so it looks as if he was a widower then.
Emily Milne McDonald, 99, died in Rosehearty in 1968.
What you need to do is go to
www.scotlandspeople.gov.uk, buy some credits, and use them to look at the originals of some or all of these births, deaths, marriages and census. (You won't be able to view births after 1919, marriages after 1944 and deaths after 1969 because those are not available online.)
Then come back to us and tell us the names of the parents of William Davidson, Mary Cruickshank, John McDonald and Emily Milne and we'll see where to go from there.