Hi there, thanks for this wonderful forum! I hope you can answer my question, because I've been searching everywhere online for answers and can't find any!

I am renting in a place with an old Weil McLain 68 Oil Boiler. It is the water heater as well as the heat for the house and it has no water storage tank, it is on-demand. We keep the place pretty cool around 60 degrees.
1. I've read that it is most efficient to keep the water temperature at the lowest setting (lo 120, hi 140), first is this true for on-demand heaters like mine that also heats the house? I also just read that you should NOT keep on-demand type heaters on the lowest setting since it's more energy to keep them at a low temperature.
Can you explain this?

Also what is the differential? Is it important to make sure it is the exact difference between the high and low?

Lastly, As I've said, I've been keeping the temperature settings on the lowest setting. With this setting I cannot get a hot shower. To get a shower I briefly turn the temperature gauge up to the highest setting and it seems to generally have enough hot water to have a 5-10 minute shower. Is this efficient? Is it most efficient to have it on the highest setting at all times for the type of water heater/heating system I have since there is no storage tank?

Thank you SO much if you can give me any insight to these questions that have been boggling my mind!!