Unless you change the oil again within ,000 miles, some of the flush can remain in the oil system compromising the lubricity of the engine oil. It can also dislodge bits of carbon that then lodge somewhere else and block an oilway.

Not a good idea to use an engine flush routinely, but if an engine has been under-serviced (particularly a diesel), it can make sense to add oil flush prior to heating up the engine, and draining the old oil.

Then change the oil again after 1,000 miles. Never ever use an engine flush and siphon the old oil out. Never siphon oil out anyway unless the engine is in a boat and you can't drain it any other way.