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I think there is probably a misunderstanding about MULTI-VISCOSITY motor oils... 5w40 flows like 5w ("winter" weight) at very low temperatures for easy starting in low temps, but thins ONLY to the viscosity equal to 40 at (the same) high temps... In other words, it REMAINS at about the SAME viscosity through a WIDE temp range... I have found this oil to lubricate well at all temps, and that it helps with the oil leakage problems these older Simplex heads are notorious for, because of the bearing surfaces that are part of the main frames... At normal room temps, it seems to fall in the 10 to 20 viscosity range, so I have no qualms about using it...

5 is thinner than 40
5 is way to thin for a XL and 40 is way to thick
the machine by nature does leak and an oil that is thick enough to not leak is too thick

I tend to give more credit to the original engineers that designed the machine in the first place as to the parts clearances involved

No booth is ever going to be anywhere near the wide temp range an automobile engine is subjected to... Perhaps in the very coldest weather a booth might get down to 50 degrees F when the theater is closed overnight, and in the hottest weather it might get to near 100 degrees F at the end of the night from the buildup of heat from the machines... Then you have to add friction heat and heat from the light source, so the oil in the film head might reach 120-130 degrees F... The 5w40 will ACTUALLY be about the same viscosity of SAE 10-20 is (at 70 degrees room temp) over this wider temp range... Regular SAE 10 at 120-130 degrees is going to be considerably thinner than SAE 5w at 70 degrees (room temp) so it will leak much more when hot... The point is that MULTI-VISCOSITY oils maintain a more constant viscosity over a wide temp range, while regular SAE grades thicken and thin widely according to the temperatures they are subjected to... SAE 10 is like molasses at sub-zero temps, and thin as water at mid-July temps... Multi-viscosity motor oil has only been available since the mid-1950s, and synthetic oil since the 1980s, but many of these machines were designed years before that... No reason to NOT use the best oil available, even if it was developed later...

No reason indeed but one should remember that the metal parts also expand when hot and the clearences get less limiting lubrcant flow which was why many of the oils getting thinner when hot was a good thing

Not so many years ago I was witness to a costly lesson for a theatre circuit that chose to use a synthetic oil across there circuit that the manufacturer of the oil claimed would be better for the machine as it was a multiweight oil it would leak out less and handle the temperature changes in the machine better
Well after replacing 30 Victoria 8 movements 3 weeks later we went back to the original straight oil and never had another issue

I have had no contact with Victoria equipment, and have no idea what type or brand oil that circuit tried, but after such an experience I would certainly be convinced to NOT use that oil in Victoria equipment...

On the other hand, after most of a lifetime in automotive sales and service, and several years experience in maintaining a 40+ unit school bus fleet, I have had only positive results using certain brands of multi-vis motor oils... That good result has also carried over to personal use of these same oils in boats, lawn mowers, antique autos, and even the Simplex film heads I mentioned... Heat, friction, and wear are always something ANY machine is subject to, and whatever lubricant is superior under these conditions seems the logical choice...

As specified oils are easily available, I've always gone along with manufacturers' specs in my projectors. It's just not an area in which I'm particularly interested in experimenting.

On the other hand, a good friend of mine has NEVER purchased projector oil. In the 40+ years I've known him, he's always used locally purchased motor oil in his Century and Wenzel intermittents. Two of his heads have been running since 1947 and are absolutely rock-solid. The Wenzels were purring like kittens until the drive-in was automated and a Century was installed there (which is now on the same oil). The machines aren't showing any indications of wear after nearly 60 years of service, and could easily be replaced for less than the cumulative difference in cost between oils.

Note that this isn't an argument for going this route, but considering how anal some people get over the subject, this particular example has always been interesting to me.