diff --git a/doc/rst/source/explain_core.rst_ b/doc/rst/source/explain_core.rst_
index 71af45a25f0..b427144f76e 100644
--- a/doc/rst/source/explain_core.rst_
+++ b/doc/rst/source/explain_core.rst_
@@ -1,2 +1,4 @@
 **-x**\ [[-]\ *n*] :ref:`(more ...) <core_full>`
-    Limit number of cores used in multi-threaded algorithms (OpenMP required).
+    Limit number of cores used in multi-threaded algorithms. Multi-threaded behavior is enabled by default.
+    That covers the modules that implement the OpenMP API (required at compiling stage) and GThreads (Glib)
+    for the grdfilter module.
diff --git a/src/gmt_dev.h b/src/gmt_dev.h
index 1f8e32fd920..8de3861cfdf 100644
--- a/src/gmt_dev.h
+++ b/src/gmt_dev.h
@@ -154,6 +154,7 @@ struct GMT_CTRL; /* forward declaration of GMT_CTRL */
 #include "gmt_nan.h"            /* Machine-dependent macros for making and testing NaNs */
 #include "gmt_error.h"          /* Only contains error codes */
 #include "gmt_synopsis.h"       /* Only contains macros for synopsis lines */
+#include "gmt_glib.h"           /* Make the GMT_xg_OPT define visible, even if HAVE_GLIB_GTHREAD is not defined */
 #include "gmt_version.h"        /* Only contains the current GMT version number */
 #include "gmt_project.h"        /* Define GMT->current.proj and GMT->current.map.frame structures */
 #include "gmt_grd.h"            /* Define grd file header structure */
diff --git a/src/gmt_glib.h b/src/gmt_glib.h
index e8fece8e59a..ec13ae4019e 100644
--- a/src/gmt_glib.h
+++ b/src/gmt_glib.h
@@ -30,6 +30,7 @@
    These are used only GLIB based multi-threading */
 
 #ifndef GMT_GLIB_H
+#define GMT_GLIB_H
 
 #ifdef HAVE_GLIB_GTHREAD
 #include <glib.h>
diff --git a/src/gmt_init.c b/src/gmt_init.c
index 75b3fe1e1b3..207d3ed6d1b 100644
--- a/src/gmt_init.c
+++ b/src/gmt_init.c
@@ -2607,8 +2607,8 @@ GMT_LOCAL int gmtinit_parse_U_option (struct GMT_CTRL *GMT, char *item) {
 }
 
 /*! -x[[-]<ncores>] */
+GMT_LOCAL int gmtinit_parse_x_option (struct GMT_CTRL *GMT, char *arg) {	/* Only effective if MP is enabled */
 #ifdef GMT_MP_ENABLED
-GMT_LOCAL int gmtinit_parse_x_option (struct GMT_CTRL *GMT, char *arg) {
 	GMT->common.x.active = true;
 	if (!arg) return (GMT_PARSE_ERROR);	/* -x requires a non-NULL argument */
 	if (arg[0] == '\0')	/* Use all processors */
@@ -2622,9 +2622,9 @@ GMT_LOCAL int gmtinit_parse_x_option (struct GMT_CTRL *GMT, char *arg) {
 		GMT->common.x.n_threads = MAX(gmtlib_get_num_processors() - abs (GMT->common.x.n_threads), 1);		/* Max-n but at least one */
 	if (GMT->current.setting.max_cores)	/* Limit to max core defaults setting */
 		GMT->common.x.n_threads = GMT->current.setting.max_cores;
+#endif
 	return (GMT_NOERROR);
 }
-#endif
 
 /*! . */
 GMT_LOCAL int gmtinit_parse_colon_option (struct GMT_CTRL *GMT, char *item) {
@@ -8124,9 +8124,9 @@ void gmtlib_explain_options (struct GMT_CTRL *GMT, char *options) {
 
 #if defined(GMT_MP_ENABLED)
 		case 'y':	/* Number of threads (reassigned from -x in GMT_Option) */
-			if (strlen (GMT_x_OPT) > 1) {	/* Only print this if it is in fact available */
+			if (strlen(GMT_x_OPT) > 1 || strlen(GMT_xg_OPT) > 1) {	/* Only print this if it is in fact available */
 				cores = gmtlib_get_num_processors();
-				GMT_Usage (API, 1, "\n%s", GMT_x_OPT);
+				(strlen(GMT_x_OPT) > 1) ? GMT_Usage(API, 1, "\n%s", GMT_x_OPT) : GMT_Usage(API, 1, "\n%s", GMT_xg_OPT);
 				GMT_Usage (API, -2, "Limit the number of cores used in multi-threaded algorithms [Default uses all %d cores]. "
 					"If <n> is negative then we select (%d - <n>) cores (or at least 1).", cores, cores);
 			}
@@ -9151,9 +9151,12 @@ int gmt_default_error (struct GMT_CTRL *GMT, char option) {
 		case 's': error += GMT->common.s.active == false; break;
 		case 't': error += GMT->common.t.active == false; break;
 		case 'w': error += GMT->common.w.active == false; break;
-#if defined(GMT_MP_ENABLED)
-		case 'x': error += GMT->common.x.active == false; break;
+		case 'x': error += GMT->common.x.active == false;
+#if !defined(GMT_MP_ENABLED)
+		error --;
+		GMT_Report (GMT->parent, GMT_MSG_INFORMATION, "Option -x: GMT is not compiled with parallel support. Only one core is used\n");
 #endif
+		break;
 		case ':': error += GMT->common.colon.active == false; break;
 
 		default:
@@ -18792,12 +18795,13 @@ int gmt_parse_common_options (struct GMT_CTRL *GMT, char *list, char option, cha
 			error += gmt_M_more_than_once (GMT, GMT->common.w.active) || gmtinit_parse_w_option (GMT, item);
 			break;
 
-#ifdef GMT_MP_ENABLED
 		case 'x':
 			error += (gmt_M_more_than_once (GMT, GMT->common.x.active) || gmtinit_parse_x_option (GMT, item));
 			GMT->common.x.active = true;
-			break;
+#if !defined(GMT_MP_ENABLED)
+			GMT_Report (GMT->parent, GMT_MSG_WARNING, "Option -x: GMT is not compiled with parallel support. Only one core is used\n");
 #endif
+			break;
 
 		case ':':
 			error += (gmt_M_more_than_once (GMT, GMT->common.colon.active) || gmtinit_parse_colon_option (GMT, item));
diff --git a/src/grdfilter.c b/src/grdfilter.c
index 61e450e4966..916c649acea 100644
--- a/src/grdfilter.c
+++ b/src/grdfilter.c
@@ -47,7 +47,7 @@ Use option -x to set the number of threads. e.g. -x2, -x4, ... or -xa to use all
 
 #include "gmt_dev.h"
 #include "longopt/grdfilter_inc.h"
-#include "gmt_glib.h"
+//#include "gmt_glib.h"		/* Now included in gmt_dev.h */
 
 #define THIS_MODULE_CLASSIC_NAME	"grdfilter"
 #define THIS_MODULE_MODERN_NAME	"grdfilter"
@@ -828,7 +828,7 @@ static int parse (struct GMT_CTRL *GMT, struct GRDFILTER_CTRL *Ctrl, struct GMT_
 						Ctrl->F.rect = true;
 					}
 					else
-						Ctrl->F.width = grdfilter_get_filter_width (API, Ctrl, &txt[1]);
+						Ctrl->F.width = grdfilter_get_filter_width (API, Ctrl, &txt[1]);	/* This has an undocumented option of reading a grid */
 					if (Ctrl->F.width < 0.0) {	/* Old-style specification for high-pass filtering */
 						if (gmt_M_compat_check (GMT, 5)) {
 							GMT_Report (API, GMT_MSG_COMPAT,
@@ -893,6 +893,20 @@ static int parse (struct GMT_CTRL *GMT, struct GRDFILTER_CTRL *Ctrl, struct GMT_
 		}
 	}
 
+#ifdef HAVE_GLIB_GTHREAD
+	/* Make the default equal to the OMP case where we use all threads if not stated otherwise. */
+	if (Ctrl->F.varwidth) {			/* Variable filter width. This option is currently undocumented. */
+		if (GMT->common.x.active)
+			GMT_Report (GMT->parent, GMT_MSG_WARNING, "Sorry, variable filter width does not support multiple threads. Reset to 1.\n" );
+		GMT->common.x.n_threads = 1;
+		GMT->common.x.active = true;
+	}
+	if (!GMT->common.x.active) {
+		GMT->common.x.n_threads = API->n_cores;
+		GMT_Report (GMT->parent, GMT_MSG_INFORMATION, "Enable all available threads (up to %d)\n", API->n_cores);
+	}
+#endif
+
 	n_errors += gmt_M_check_condition (GMT, !Ctrl->G.active, "Option -G: Must specify output file\n");
 	n_errors += gmt_M_check_condition (GMT, !Ctrl->In.file, "Must specify input file\n");
 	n_errors += gmt_M_check_condition (GMT, !Ctrl->D.active, "Option -D: Choose from p or 0-5\n");
diff --git a/src/potential/gmtgravmag3d.c b/src/potential/gmtgravmag3d.c
index 2ac7b746ccd..1c2da2a8828 100644
--- a/src/potential/gmtgravmag3d.c
+++ b/src/potential/gmtgravmag3d.c
@@ -252,7 +252,7 @@ static int usage (struct GMTAPI_CTRL *API, int level) {
 	return (GMT_MODULE_USAGE);
 }
 
-static int parse (struct GMT_CTRL *GMT, struct GMTGRAVMAG3D_CTRL *Ctrl, struct GMT_OPTION *options) {
+static int parse(struct GMT_CTRL *GMT, struct GMTGRAVMAG3D_CTRL *Ctrl, struct GMT_OPTION *options) {
 
 	/* This parses the options provided to gmtgravmag3d and sets parameters in Ctrl.
 	 * Note Ctrl has already been initialized and non-zero default values set.
diff --git a/src/potential/grdgravmag3d.c b/src/potential/grdgravmag3d.c
index 9ca9129ebc6..168384fb56c 100644
--- a/src/potential/grdgravmag3d.c
+++ b/src/potential/grdgravmag3d.c
@@ -262,7 +262,7 @@ static int usage (struct GMTAPI_CTRL *API, int level) {
 	return (GMT_MODULE_USAGE);
 }
 
-static int parse (struct GMT_CTRL *GMT, struct GRDGRAVMAG3D_CTRL *Ctrl, struct GMT_OPTION *options) {
+static int parse(struct GMT_CTRL *GMT, struct GRDGRAVMAG3D_CTRL *Ctrl, struct GMT_OPTION *options) {
 
 	/* This parses the options provided to grdgravmag3d and sets parameters in Ctrl.
 	 * Note Ctrl has already been initialized and non-zero default values set.
diff --git a/src/surface.c b/src/surface.c
index 9dc34f3700f..28ee64cc616 100644
--- a/src/surface.c
+++ b/src/surface.c
@@ -57,7 +57,7 @@
 #define THIS_MODULE_PURPOSE	"Grid table data using adjustable tension continuous curvature splines"
 #define THIS_MODULE_KEYS	"<D{,DD(=,LG(,GG}"
 #define THIS_MODULE_NEEDS	"R"
-#define THIS_MODULE_OPTIONS "-:RVabdefhiqrw" GMT_ADD_x_OPT GMT_OPT("FH")
+#define THIS_MODULE_OPTIONS "-:RVabdefhiqrw" GMT_OPT("FH")
 
 struct SURFACE_CTRL {
 	struct SURFACE_A {	/* -A<aspect_ratio> */
@@ -1790,7 +1790,7 @@ static int usage (struct GMTAPI_CTRL *API, int level) {
 	GMT_Usage (API, -2, "Change over-relaxation parameter [Default = %g]. Use a value "
 		"between 1 and 2. Larger number accelerates convergence but can be unstable. "
 		"Use 1 if you want to be sure to have (slow) stable convergence.", SURFACE_OVERRELAXATION);
-	GMT_Option (API, "a,bi3,di,e,f,h,i,qi,r,w,x,:,.");
+	GMT_Option (API, "a,bi3,di,e,f,h,i,qi,r,w,:,.");
 	if (gmt_M_showusage (API)) GMT_Usage (API, -2, "Note: Geographic data with 360-degree range use periodic boundary condition in longitude. "
 		"For additional details, see Smith & Wessel, Geophysics, 55, 293-305, 1990.");