Now I have done a bit more testing and seen your screen shot, I am not so sure about the minimum limit as your tankers are initially transferring 500 at a time whereas mine are doing 1000 at a time.
One test I have done is as follows using four tankers with a capacity of 110000 each:
Set the Crude queue size to 1000
Set approximate unload time to be 18 seconds
Set consumption of Crude to 1000 every 6 seconds
Results:
10:01:41 Hrs. Tankers started unloading 1000 at a time from only one tanker at a time
10:16:53 Hrs. Tankers started unloading 500 at a time from two tankers (sometimes it was from the same tanker but two unloads)
11:02:56 Hrs. and the tankers are still unloading
The remaining amount of Crude in the tankers at this time is 64499, 64499, 62000 and 63000 (so a long time to go before finished unloading)
Test aborted (by me) early
The next test was done exactly the same but the Crude queue was set to 1,000,000
Results:
10:01:22 Hrs. Tankers started unloading 1000 at a time from each tanker one at a time
10:03:13 Hrs. All tankers have been unloaded
The final test I did was to remove the processing and use industry link to move the Crude to another industry with both Crude queues set to be 1,000,000
Results:
10:01:22 Hrs. Tankers started unloading 1000 at a time from each tanker one at a time
10:01:29 Hrs. Industry Link transferred 41000 to second industry with tankers still unloading
10:01:39 Hrs. Another 59000 transferred to second industry with tankers still unloading
10:01:49 Hrs. Another 10000 transferred to second industry with tankers still unloading
10:01:59 Hrs. Another 30000 transferred to second industry with tankers still unloading
10:02:09 Hrs. Another 59000 transferred to second industry with tankers still unloading
10:02:19 Hrs. Another 21000 transferred to second industry with tankers still unloading
10:02:29 Hrs. Another 17000 transferred to second industry with tankers still unloading
10:02:39 Hrs. Another 60000 transferred to second industry with tankers still unloading
10:02:49 Hrs. Another 33000 transferred to second industry with tankers still unloading
10:02:59 Hrs. Another 13000 transferred to second industry with tankers still unloading
10:03:09 Hrs. Another 59000 transferred to second industry with tankers still unloading
10:03:15 Hrs. Tankers finished unloading
10:03:19 Hrs. Another 38000 transferred to second industry
Not really sure what to make of all that other than there are ways to make the unload time take longer by fiddling with the queue sizes, and/or process and unload times