I looked at longer drain OCI’s in my GM Holden Commodore V6 3800, ie Buick V6 3800 in US, and I decided against it.
Holden went to longer drains in 2004, extending the OCI’s from 6 months to 9 months or 10000km to 15000km and the sump size was increased from 4.8L to 5.6L.
The new Global GM Holden V6 3600 HFV6 engine, which is also used in GM cars world wide, has had the sump capacity increased to 6.5L and the OCI's have been increased to 12 months or 15000 km.
Both V6 engines ie the old 3800 and the new 3600 are recommended by Holden to use the 10w30 grade for normal use.
Based on these observations I was reluctant to extend OCI's beyond 6 months or 10000 km as my sump capacity is only 4.3L in my 1995 model. You can see the sump capacity has gone from 4.3L (1995) to 6.5L (2006) ie approx. 50% increase.
This indicates to me there is a correlation between OCI and sump capacity, in the absence of running regular used oil analysis to check the performance of the oil, in engines that were not designed by the OEM for longer OCI's.