I stopped using MST after my oil light came on a few times on my last NT trip, it was only when the temp reached 3/4 (which is not often) but it happened a few times.
I believe it thinned out to much at those temps.
Spoke to Penrite about using a thicker grade oil (10W-40) but when I said oil usage was minimal (about 500ml between oil changes) they recommended staying with a 5W-40.
Now the most interesting part is they thought ALL WH's had a particulate filter, when I told them mine didn't they recommended I try a 229.5 (not .51) as its a full zinc oil & will provide better protection.
I am currently running HRP 5 5W-40.
https://www.autobarn.com.au/penrite-...-5w-40-oa02687