Hi,
the answers on sump capacity have been eloquently and accurately given by others in here but I can answer regarding OCIs
Stroker444 - Depending on use, the engine family involved and the lubricant's specification, (and using UOAs) these general figures will give you an insight (similar Approvals and specification in each case)
1 - Manufacturer Approved mineral lubricants (HDEO 15w-40) can typically go about 50% beyond "normal" baseline OC point
2 - Manufacturer Approved semi-synthetic (Group 3) (HDEO 15w-40) will typically go to double the baseline and perhaps a little more
3 - Manufacturer Approved synthetic (Group 3/4/5) (HDEO 5w-40)
will typically go to 120% of the baseline
That said, typically the main issue in condemning a lubricant in these engines is usually the soot level and in many lubricants in both types 1 & 2 above, a significant "out of range" regarding viscosity. Sometimes it is also the iron level (usually around 150ppm)
Some lubricants of type 3) above handle soot better than all the others
Some brands in 1) above such as Delvac, Delo and Rotella/Rimula have quite amazing abilities in keeping engines clean and ensuring a long life. This has been the way for at least 50 years or so
You mention Delvac 1300 15w-40. It is really a hidden gem in soot control. Many years ago I ran comparative tests of four lubricants in the Southern Alps of NZ and this was the star performer even then. The vehicles were high utililisation 24/7 Dozers and Scrapers
In Linehaul/Interstate trucking the positive economics in using a full synthetic lubricant only accrue in extending the OCI via UOAs. It simply becomes an economics issue including any upside in vehicle availability of course
Using the likes of a centrigfuge oil cleaner (as distinct from a cartridge type by-pass filter) and a first class synthetic lubricant from a Major Oil Company it is possible to regularly extend out to six or seven times the engine manufacturer's recommended OCI
Regards
Doug