This synthesis can be adapted to the laboratory by heating oleum to slowly distill the sulfur trioxide into a cooled flask of sulfur dichloride.

SO3 +SCl2=SOCl2 + SO2

The major industrial synthesis involves the reaction of sulfur trioxide and sulfur dichloride: this synthesis can be adapted in the laboratory by heating the oleum to slowly distill the sulfur trioxide in a cold flask of sulfur dichloride. This synthesis can be adapted to the laboratory by heating oleum to slowly distill the sulfur trioxide into a cooled flask of sulfur dichloride.The major industrial synthesis involves the reaction of sulfur trioxide and sulfur dichloride

If you are able to purchase thionyl chloride, the idea is to distill it right before use.

This can be done by distillation from triphenyl phosphite - see

https://pubs.rsc.org/en/content/articlelanding/1967/j1/j19670000036

or the older method - distillation from quinoline - see

https://www.researchgate.net/post/Can-any-one-suggest-an-efficient-method-to-distill-thionyl-chloride