"---MIKE---" wrote
I have a small log home. The logs are efficient insulators plus they
act as a heat sink. My heat is baseboard hot water and I have two
thermostats controlled by a homemade switcher. The "day" thermostat is
set to 65° and the night one is 57°. This does save some oil. My
question is about daytime if I leave the house. Does it make sense to
go on the lower setting for just four hours? It would use less oil
during that time but then it would use more to "catch up" when I get
home. What would be the minimum number of hours that would give the
best savings on oil?

It would depend on how much drop in temp you get over 1 hour, and 4 hours.

2 scenarios he

1- when you shut down at night to 57, it takes 3 hours or more before the
temp hits 57 but in the morning when you bring it up, it rewarms pretty much
in 15 mins or less.
- may not save you that much

2- when you shut down at night to 57, it takes 1 hour or less to hit that
temp and takes at least 15 mins, maybe more to rewarm.
- will save you money