There is no real problem with this, except that you increase the risk of conflicting update on both sides. Note that doing this it will not completely solve your problem, only reduce it. Let say the sync is set to 30 minutes and starts at 9:00 and 9:30. If a sales starts editing a lead at 9:28, you will still hit the same error message.
I would rather use some alternatives, such as providing another custom object in SFDC and let the sales edit this object, to log the call, then, when this object is saved, push the updates to the lead (This could be done simply with workflows or using a flow for the user interface).