1939 Sq Four 599cc Engine Oil

[James_McD]

Hi All, my brother and I have just bought a 1939 Square Four 4F 599cc and are having it recommissioned locally near Oxford. the guys working on it have a question about engine oil and do you have any recommendations. They have heard talk of maybe a synthetic oil is preferred due to the heat of the rear cylinders - is this correct and does anyone recommend an oil?
Thanks

[Paul_Linden]

Well done on buying such a nice bike - it’s always good to post a picture or two on this forum so we can all enjoy looking at it.
The question of “which oil for my bike” always generates lots of discussion on online groups for old Brit bikes.
I believe your bike has white metal in the big ends and mains, and also can suffer from wear on the cam lobes and followers.
As such you could consider Penrite classic medium, a 25W/70 oil with zinc compounds to mitigate cam wear.
It’s what I use in my Mark 2 - here is some info about it - I’m NOT an agent for the company:
https://www.classic-oils.net/Penrite-Classic-Medium
Whatever modern oil you use, it will be better than what was available at the time it was built.
Do make sure the sludge traps have been cleaned, and it’s a good idea to fit an oil filter.

[JohnnyBeckett]

hi i think the oil i would use castrol GP 50 or XXL 40 oil or you can use duckhams 20/50 mineral oil on a sq four ,, do not use a synthetic oil

[paul.jameson]

The reference to "heat of the rear cylinders" suggests strongly to me that your recomissioners are non too experienced in Square Fours. It is absolutely essential to ensure that the crankshaft sludge traps are clean and that all old oily residue has been removed from the engine internals before you start to use the bike. Otherwise, the engine will blow up with a con-rod through the crankcases in about 500 miles.
The particular oil used after the cleaning process is an unimportant detail.

[nevhunter]

Unless you can absolutely guarantee that the crankshaft(s) oil passages have had the plugs removed and oilways thoroughly cleaned, you must redo it.. The same applies to the twins. but they have simpler oil pathways...Nev

[James_McD]

thanks for all the replies guys - will defo post some pics when I next visit her, should be next week. can't wait top get out on the road.,