diff --git a/Frameworks/Account/Account.swift b/Frameworks/Account/Account.swift
index 6d74cd7a2..f6c069760 100644
--- a/Frameworks/Account/Account.swift
+++ b/Frameworks/Account/Account.swift
@@ -542,7 +542,7 @@ public final class Account: DisplayNameProvider, UnreadCountProvider, Container,
 
 		"""
 
-		let middleText = OPMLString(indentLevel: 0)
+		let middleText = OPMLString(indentLevel: 0, strictConformance: false)
 
 		let closingText =
 		"""
@@ -1079,6 +1079,7 @@ private extension Account {
 			}
 
 			if let folder = ensureFolder(with: folderName) {
+				folder.externalID = item.attributes?["nnw_externalID"] as? String
 				if let itemChildren = item.children {
 					loadOPMLItems(itemChildren, parentFolder: folder)
 				}
@@ -1160,13 +1161,13 @@ extension Account {
 
 extension Account: OPMLRepresentable {
 
-	public func OPMLString(indentLevel: Int) -> String {
+	public func OPMLString(indentLevel: Int, strictConformance: Bool) -> String {
 		var s = ""
 		for feed in topLevelFeeds {
-			s += feed.OPMLString(indentLevel: indentLevel + 1)
+			s += feed.OPMLString(indentLevel: indentLevel + 1, strictConformance: strictConformance)
 		}
 		for folder in folders! {
-			s += folder.OPMLString(indentLevel: indentLevel + 1)
+			s += folder.OPMLString(indentLevel: indentLevel + 1, strictConformance: strictConformance)
 		}
 		return s
 	}
diff --git a/Frameworks/Account/Feed.swift b/Frameworks/Account/Feed.swift
index 058bafdfc..397753161 100644
--- a/Frameworks/Account/Feed.swift
+++ b/Frameworks/Account/Feed.swift
@@ -226,7 +226,7 @@ public final class Feed: DisplayNameProvider, Renamable, UnreadCountProvider, Ha
 
 extension Feed: OPMLRepresentable {
 
-	public func OPMLString(indentLevel: Int) -> String {
+	public func OPMLString(indentLevel: Int, strictConformance: Bool) -> String {
 		// https://github.com/brentsimmons/NetNewsWire/issues/527
 		// Don’t use nameForDisplay because that can result in a feed name "Untitled" written to disk,
 		// which NetNewsWire may take later to be the actual name.
diff --git a/Frameworks/Account/Folder.swift b/Frameworks/Account/Folder.swift
index bdfe13118..9cfd52e18 100644
--- a/Frameworks/Account/Folder.swift
+++ b/Frameworks/Account/Folder.swift
@@ -24,6 +24,7 @@ public final class Folder: DisplayNameProvider, Renamable, Container, UnreadCoun
 	
 	static let untitledName = NSLocalizedString("Untitled ƒ", comment: "Folder name")
 	public let folderID: Int // not saved: per-run only
+	public var externalID: String? = nil
 	static var incrementingID = 0
 
 	// MARK: - DisplayNameProvider
@@ -136,20 +137,29 @@ private extension Folder {
 
 extension Folder: OPMLRepresentable {
 
-	public func OPMLString(indentLevel: Int) -> String {
+	public func OPMLString(indentLevel: Int, strictConformance: Bool) -> String {
+		
+		let attrExternalID: String = {
+			if !strictConformance, let externalID = externalID {
+				return " nnw_externalID=\"\(externalID)\""
+			} else {
+				return ""
+			}
+		}()
+		
 		let escapedTitle = nameForDisplay.rs_stringByEscapingSpecialXMLCharacters()
-		var s = "<outline text=\"\(escapedTitle)\" title=\"\(escapedTitle)\">\n"
+		var s = "<outline text=\"\(escapedTitle)\" title=\"\(escapedTitle)\"\(attrExternalID)>\n"
 		s = s.rs_string(byPrependingNumberOfTabs: indentLevel)
 
 		var hasAtLeastOneChild = false
 
 		for feed in topLevelFeeds  {
-			s += feed.OPMLString(indentLevel: indentLevel + 1)
+			s += feed.OPMLString(indentLevel: indentLevel + 1, strictConformance: strictConformance)
 			hasAtLeastOneChild = true
 		}
 
 		if !hasAtLeastOneChild {
-			s = "<outline text=\"\(escapedTitle)\" title=\"\(escapedTitle)\"/>\n"
+			s = "<outline text=\"\(escapedTitle)\" title=\"\(escapedTitle)\"\(attrExternalID)/>\n"
 			s = s.rs_string(byPrependingNumberOfTabs: indentLevel)
 			return s
 		}
diff --git a/Mac/Scriptability/Account+Scriptability.swift b/Mac/Scriptability/Account+Scriptability.swift
index 520c3d298..2b2bdac09 100644
--- a/Mac/Scriptability/Account+Scriptability.swift
+++ b/Mac/Scriptability/Account+Scriptability.swift
@@ -150,7 +150,7 @@ class ScriptableAccount: NSObject, UniqueIdScriptingObject, ScriptingObjectConta
 
     @objc(opmlRepresentation)
     var opmlRepresentation:String  {
-        return self.account.OPMLString(indentLevel:0)
+        return self.account.OPMLString(indentLevel:0, strictConformance: true)
     }
 
     @objc(accountType)
diff --git a/Mac/Scriptability/Feed+Scriptability.swift b/Mac/Scriptability/Feed+Scriptability.swift
index 27a1aa791..f2ae486e5 100644
--- a/Mac/Scriptability/Feed+Scriptability.swift
+++ b/Mac/Scriptability/Feed+Scriptability.swift
@@ -146,7 +146,7 @@ class ScriptableFeed: NSObject, UniqueIdScriptingObject, ScriptingObjectContaine
 
     @objc(opmlRepresentation)
     var opmlRepresentation:String  {
-        return self.feed.OPMLString(indentLevel:0)
+        return self.feed.OPMLString(indentLevel:0, strictConformance: true)
     }
     
     // MARK: --- scriptable elements ---
diff --git a/Mac/Scriptability/Folder+Scriptability.swift b/Mac/Scriptability/Folder+Scriptability.swift
index a5ea88618..8f522513c 100644
--- a/Mac/Scriptability/Folder+Scriptability.swift
+++ b/Mac/Scriptability/Folder+Scriptability.swift
@@ -101,7 +101,7 @@ class ScriptableFolder: NSObject, UniqueIdScriptingObject, ScriptingObjectContai
 
     @objc(opmlRepresentation)
     var opmlRepresentation:String  {
-        return self.folder.OPMLString(indentLevel:0)
+        return self.folder.OPMLString(indentLevel:0, strictConformance: true)
     }
 
 }
diff --git a/Shared/Exporters/OPMLExporter.swift b/Shared/Exporters/OPMLExporter.swift
index c177431ef..75dde22d5 100644
--- a/Shared/Exporters/OPMLExporter.swift
+++ b/Shared/Exporters/OPMLExporter.swift
@@ -27,7 +27,7 @@ struct OPMLExporter {
 			
 			"""
 
-		let middleText = account.OPMLString(indentLevel: 0)
+		let middleText = account.OPMLString(indentLevel: 0, strictConformance: true)
 
 		let closingText =
 			"""
diff --git a/submodules/RSCore b/submodules/RSCore
index b3a3178b0..d70c24feb 160000
--- a/submodules/RSCore
+++ b/submodules/RSCore
@@ -1 +1 @@
-Subproject commit b3a3178b00810c9e2c668a8c3b17080501cf1f33
+Subproject commit d70c24feb4bdad8fdbe7e0beb6edd12d3383c7dc