Nigel Hughes has carried out a very detailed survey looking into the question of suitable oils and their viscosities. Here are his findings:
For engine oils, early pre-1940 handbooks generally recommend Price’s Motorine C, but later handbooks extend the list to include Wakefield’s Castrol XXL, Vacuum Mobiloil BB, Shell Triple or Duckham’s Adcoidised N.P.3. These were all what we would today recognise as single SAE viscosity grade 40.
However, by November1946, Service Leaflet R-R D2 revised the recommendations for the older models, from Phantom I onwards, to an SAE 30 oil: BP Energol 30, Castrol XL, Shell X-100 30 or Mobil Mobiloil A. If these were unavailable, Duckham’s ‘Thirty’ NOL, Esso Essolube 30, Gulfpride Motor 30 or Gulflube Motor Oil 30 or Sternol w.w. 30 were to be used. If oil consumption was high, an SAE 40 oil was recommended and in winter an SAE 20.
Possibly with the exception of Shell X-100, all these oils were non-detergent, meaning that they did little to keep an engine clean.
In the 1950s, multigrade oils were introduced. In simple terms, these behaved as a thinner winter grade when cold, aiding starting and markedly reducing start-up wear, but had the viscosity of a summer grade when hot. Initially these oils were often graded as SAE 10W-30, though in the 1960s 20W-50 oils became popular particularly for Minis, where the engine and gearbox shared the same oil, and for reducing oil consumption in higher mileage engines. These multigrade oils contained ‘new’ detergent and dispersant additives for reducing sludge formation and for keeping engines clean. When put into old engines which had become dirty from use of non-detergent oils, they tended to scour out dirt leading to blocked oilways and filters with disastrous consequences.
If your engine has not had a thorough clean out of oilways, particularly the ‘sludge traps’ in the crankshaft, or is a new acquisition of unknown condition, you should not use a high detergency oil. There are several sources of single grade ‘classic’ oil of low or nil detergency.
If your engine is clean internally or has been running on multigrade oil for years and has had annual or 1000 mile oil changes, you can use a ‘classic’ 20-50 multigrade. Unless your engine has been fitted with a modern style full-flow oil filter, you should still use a 1000 mile oil change interval as the dispersant in the oil will be keeping sludge and other particles in suspension, to be removed by oil change, rather than by the filter. Suitable ‘classic’ multigrades are available from several sources.
Regardless of the condition of your engine, you must avoid modern thin oils with grades such as 0W-30. These are totally unsuitable for the materials and bearing tolerances of any pre-1940 engine. From available information, it appears that all ‘Classic’ oils contain beneficial amounts of anti-wear additive ZDDP, whether single grade or multigrade.
Some sources of suitable engine oils are:
https://www.morrislubricantsonline.co.uk/scripts/default.asp
https://www.castrol.com/castrol/castrolhomepage.do?categoryId=3205&contentId=6031662
https://www.penriteoil.com/uk/nextpage.php?navlink=
https://www.millersoils.net/
https://www.opieoils.co.uk/c-664-monograde-classic-oils.aspx
Viscosities 40C 100C
Millerol 30 93.9 11.2
Millerol 40 141 14
Pistoneez P30 104 11.9
Pistoneez P40 130 13.9
Morris
Golden Film 30 100 11.6
Golden Film 40 140 15
Golden Film 20/50 135 17.5
Penrite
Shelsley Light 20/30 218 23.6 Min 5C Oil change 1000 mi or yearly
Shelsley Medium 40 292 27.9 Min 10C
HPR 30 213 23.2 Min 5C replaces 20W, 30 and old 20W/50
HPR 40 282 27.2 Min 10C replaces old SAE40
Classic 20W/50 169 18.8 3000 mi oil change for HPR and Classic
Castrol
XL 30 99 11.05
XXL40 155 14.89
XL20-50 158 17.3
