Years ago I competed in several endurance events. The thinking at the time was a single weight oil..as the engines were running non-stop. It seemed to be a good decision. No oil failures.
Use Brad Penn if you can find it.
If the weather is hot +85*f..50w..if it is cool..40w.
At the end of the event..drop out the single grade oil and use a multi-weight oil for running around or sprint racing.
depending on which straight wt you go with the ambient temp at startup might be a concern.
i would look to the Maxima Ultra4 5w50 or Extra4 15w50 (i've tested this one) and then there is 530RR.
maybe some of the upper end Motul products like 300V and 5100
the newest Amsoil is finally tough enough.
and of course Valvoline VR1 20w50 or sae50 (both i have tested) could be choices as well.
lots to consider! oh and i will also ask my buddy bill that just won the CMRA unlimited classic and F40 LW class titles.