Primegrid will test your machines as they run HOT HOT HOT.
BOINC Tasks you can change the "Boinc Preference" on one machine at a time not all at once and the priciple is the same as in Boinc Manager below.
The whole idea of the Challenge is to find Prime Numbers and to do this you must be the first to process the work unit and send it back. That is why "Store at least X days of work" and "Store up to an additional X days of work" are set to "0".
The preference thing is a bit difficult. If you have never set preferences in Boinc Manager at the machine level then you are using your Global preferences that was set on on some project in the past
http://www.primegrid.com/prefs.php?subset=global and you can change this and it will change all machines the next time they update or the BOINC client is stopped and re started, not BOINC Manager as stopping and starting the manager does nothing.
However, If you have ever changed the preferences in Boinc Manager then you are using local preferences for that machine only. You will need to one of the following on all the machines where you have a local preference.
When you open the "Computing Preferences" in Boinc Manager, look at the top it will say that you aree using Local or Global.
a. If it says "Using Local preferences" then you can.
1. If you have set you Global preferences on Primegrid then click the button "Use web prefs" , or
2. If you want to keep all the local peference change the options in Boinc Manager.
b. Same as a. but the reverse
a LOCAL preference over rules a GLOBAL.
I have a different preference on each machine because I want to change the CPU % etc locally.