You don't need to do so, unless we're talking 1 platform serving all the needs. To the best of my knowledge it is possible to just set timing for the stops but not for the journey time (I suppose by setting the journey time very low).
I'm using multiple platforms. This is actually somewhat close to what I'm looking for, I didn't know you could use partial timetables. I just tested it and yes, I can assign a number of days to loading and I don't even have to use a low number for the journey, I can leave it "not timetabled".
And if you're using a single train for the whole pipeline, wouldn't it mean the raws always come before the processed ?
I use different trains. In this case, it was two trains for oil and one for goods.
Also, if you want to make one platform that serves all the needs, the trick is not to assign any "full load" order. It's not very efficient but it's the way it works I suppose (I mean, 2 platforms is more efficient).
I'm using multiple platforms so trains doing the "full load" for goods do not impede the unloading of oil. What bothers me is when the unloaded oil is not enough to produce full goods for the goods train and it has to wait for another oil delivery to complete. Like I said, it's only relevant in the early game, I now have 10+ trains delivering oil from different wells so the cadence is good enough that it doesn't bother me.