most premium EO sell in 1L bottle, so if you don't use up all the contents, you won't keep too much leftovers in unsealed condition which *may* degrade the EO's performance.

That, and it's easier to sell/can sell more premium price using 1L bottles

SR/TR etc are not "sub brands", more like "models" of their lineup.
if you want to go full synthetic for daily use, SR-1 is more than enough. TR-5 is their semi-synthetic (nothing to shout about to be honest)

so if you want to give torco a try, go with SR-1 then.

SR4/5 = racing EO, and not really suitable for daily usage in the sense that they have less detergent and not that good for long OCI. unless of course you got money to spend to blow doing shorter OCI at much more expensive pricing.