Bearing clearances, oil flow and oil choice is more complicated than the generalisations in this post may make it seem.
A general rule for this is a [1 thousands of an inch gap] for a [1 inch diameter shaft], [2 thousands of an inch gap] for a [2 inch shaft], and so on.
For those that don't deal with these small numbers that often and want to picture what that is, pluck a hair out of your head... that's about 1 or 2 thou thick.
For smaller gaps you may desire a thinner oil, for a larger gap you may want a thicker oil.
For higher stresses like high RPM and extended race sessions, you may want a larger gap... let's say about 20% more, for now.
10w30 is a fairly standard viscosity for standard engine clearances, for min film thickness reasons and other things.
Getting it wrong
It wasn't long before the bearings that go between the connecting rod and the crankshaft starting failing.
A few people stripped the engines to find that the clearance between these failing bearings (which we would expect to be approximately [1.20 thou/inch] from our rough rule-of-thumb above) was in fact about half that, around [0.61 thou/inch].
Hmm..! Rather thin gap, rather thick oil.
Some people say that thinner oil might help a little bit, some say the opposite. What seems to be agreed upon is that you should fit bearings with more clearance to solve the problem.
Oh yeah, rotaries...
Something to get out of the way initially: rotor bearings on each seem to be as expected. They only have a rough max 6,000 RPM relative to their journal on the eccentric shaft (so nothing too punishing there).
Their clearances are about [1.08 thou/inch] on both - cool.
Main bearings however, for whatever reason seem to be somewhat gaping! We have rather funny engines so this is probably nothing to be alarmed about, but it's a peculiar spot none-the-less.
RX-8 main bearing clearance: [1.74 thou/inch]
FD main bearing clearance: [1.86 thou/inch] - even looser!
How all this plays into the "5w30 oil is too thin" debate, and whether 10w40 should therefore be better ... some might think from these calculations that it is quite obvious (no doubt from how the post has been worded ).
However, it should be noted that these are just some very general 'rules' and calculations, and the physics behind it all is far more complex than I will ever be willing to understand without being paid a decent sum of money to gather and apply that knowledge.