Mantis#1673. Thank you kindly, Matth for a patch that:

LSLInteger + literal integer is not an LSLInteger.
The included patch fixes the issue: LSLInteger + literal 
integer is not an LSLInteger (also fixed for -,*,/)

OpenSim/Region/ScriptEngine/Common/LSL_Types.cs

@@ -1292,6 +1292,26 @@ namespace OpenSim.Region.ScriptEngine.Common
                 bool ret = i1.value != i2.value;
                 return ret;
             }
+            
+            static public LSLInteger operator +(LSLInteger i1, int i2)
+            {
+                return new LSLInteger(i1.value + i2);
+            }
+            
+            static public LSLInteger operator -(LSLInteger i1, int i2)
+            {
+                return new LSLInteger(i1.value - i2);
+            }
+            
+            static public LSLInteger operator *(LSLInteger i1, int i2)
+            {
+                return new LSLInteger(i1.value * i2);
+            }
+            
+            static public LSLInteger operator /(LSLInteger i1, int i2)
+            {
+                return new LSLInteger(i1.value / i2);
+            }
 
             public override bool Equals(Object o)
             {

OpenSim/Region/ScriptEngine/Shared/LSL_Types.cs

@@ -1320,6 +1320,26 @@ namespace OpenSim.Region.ScriptEngine.Shared
                 bool ret = i1.value != i2.value;
                 return ret;
             }
+            
+            static public LSLInteger operator +(LSLInteger i1, int i2)
+            {
+                return new LSLInteger(i1.value + i2);
+            }
+            
+            static public LSLInteger operator -(LSLInteger i1, int i2)
+            {
+                return new LSLInteger(i1.value - i2);
+            }
+            
+            static public LSLInteger operator *(LSLInteger i1, int i2)
+            {
+                return new LSLInteger(i1.value * i2);
+            }
+            
+            static public LSLInteger operator /(LSLInteger i1, int i2)
+            {
+                return new LSLInteger(i1.value / i2);
+            }
 
             static public LSLInteger operator &(LSLInteger i1, LSLInteger i2)
             {