What's a good oil grade and viscosity for the Isuzu diesels. Specifically my 4BD1T.

I'm currently using 15/40wt shell rimula. The only issue I've had is under sustained hard loading (i.e. 4th gear up ski-field roads) it gets so thin the oil light flickers at idle. It comes right after it's cooled again.

An oil cooler would probably fix that, but it's something I only do once a year so I can work around the problem.