We are still talking about lifter valley drainback?

The R2 and R3 blocks had no drainbackbuilt in. It's not the enclosed tunnels fault. On the R blocks you can expose the cam and the oil drains on the spinning assembly if that's OK with the builder. On R5 or the XR blocks you have to open the bottom of the cam tunnel as well with wet sump. The R5's have been converted using 3/4" holes on either side of cam bearings to drain oil.

On R block I drained around distributor and using stock drain holes to drain behind belt drive. Trick is the heads need to drain seperate with that limited drainback around lifter valley. Haven't seen that on R5 or XR. I run R5 with dry sump, haven't needed to experiment this.

If you like swiming in oil, limit your drainback and add oil to keep from starving, NOT. Don't skimp making drainback passages. Wonder if anyone ever thought of plumbing a secondary pickup plumbed to rear of cam tunnel. May be a disaster. May work.