Designated Pinch Hitter: 1995-2014.

wikipedia:

After the (1968) season, the rules were changed to lower the mound from 15 to 10 inches and change the upper limit of the strike zone from the top of a batter's shoulders to his armpits. In addition, in 1969 spring training, both the American League and National League agreed to try the designated pinch hitter (DPH), but they did not agree on the implementation.

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Víctor Martínez May 14, 2014
by Keith Allison via Wikimedia Commons

Major Baseball League (MBL) Commissioner Allan Huber "Bud" Selig moved the Houston Astros from the National Conference (NC) to the American Conference (AC) in 2013.  You can do that within a league, move a team between conferences. Selig had done that in 1998, moved the Milwaukee Brewers, which he owned at the time, from the AC to the NC.  What Selig has not done is resolve the designated hitter rule between the two conferences.  AC has it, NC does not.

With the addition of Houston two years ago the number of AC teams increased from 14 to 15.  Consider that in viewing the numbers below for players who are primarily designated hitters (DH).   I looked for players who qualified for league batting title (at least 502 plate appearances after 1995) and played at least these percents of their games at DH:  50%, 75%, 90%, 100%.  Source: baseball-reference.com

Click this link to view individual players by year.

Among the 54 players seasons in the 90% group:
Youngest were only 25:
Brad Fullmer 2000
Billy Butler 2011

Oldest 41:
Paul Molitor 1998
Edgar Martinez 2004

Home Runs:
Most: David Ortiz 2006 54; also led AC in HR, RBI, BB
Fewest: Paul Molitor 1998 4

OPS:
highest: Edgar Martinez 1995 1.107; also led AC in G, R, 2B, BA, OBP, OPS
lowest: Paul Molitor 1998 .718

The only other significant black ink (leading AC):
Travis Hafner 2006 SLG, OPS
David Ortiz:
- 2007 OBP
- 2005 RBI
Edgar Martinez:
- 2000 RBI
- 1999 1998 OBP

11 of 54 seasons OPS > 1.000
11 of 54 seasons OPS < .800.
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year	50%	75%	90%	100%
2014	6	4	1	0
2013	5	4	3	0
2012	6	3	0	0
2011	8	7	3	1
2010	7	4	1	0
2009	6	2	2	1
2008	4	2	1	1
2007	7	6	5	1
2006	5	4	4	1
2005	8	3	2	0
2004	4	4	3	1
2003	6	3	1	1
2002	3	2	2	0
2001	5	3	2	0
2000	3	3	2	0
1999	6	5	4	1
1998	8	7	3	0
1997	7	5	5	1
1996	10	7	4	2
1995	8	7	6	4
tot	122	85	54	15
max	10	7	6	4
min	3	2	1	0
ave	6.22	4.67	3.22	1.00
pct	50%	75%	90%	100%

At least 50% as DH in 2013, 2014:

Led AC:
Nelson Cruz 2014 HR
Victor Martinez 2014 OBP, OPS

Rk	Player	Year	Age	Tm	Lg	G	PA	AB	R	H	2B	3B	HR	RBI	BB	IBB	SO	HBP	SH	SF	GDP	SB	CS	BA	OBP	SLG	OPS	Pos
1	Chris Carter	2014	27	HOU	AL	145	572	507	68	115	21	1	37	88	56	6	182	5	0	4	12	5	2	.227	.308	.491	.799	*D3/H7
2	David Ortiz	2014	38	BOS	AL	142	602	518	59	136	27	0	35	104	75	22	95	3	0	6	18	0	0	.263	.355	.517	.873	*D/H3
3	Nelson Cruz	2014	33	BAL	AL	159	678	613	87	166	32	2	40	108	55	8	140	5	0	5	17	4	5	.271	.333	.525	.859	*D79
4	Billy Butler	2014	28	KCR	AL	151	603	549	57	149	32	0	9	66	41	3	96	5	0	8	21	0	0	.271	.323	.379	.702	*D3/H
5	Victor Martinez	2014	35	DET	AL	151	641	561	87	188	33	0	32	103	70	28	42	4	0	6	17	3	2	.335	.409	.565	.974	*D3/2H
6	Adam Dunn	2014	34	TOT	AL	131	511	429	49	94	18	0	22	64	71	5	159	7	0	4	5	1	1	.219	.337	.415	.752	*D3H/791
7	Billy Butler	2013	27	KCR	AL	162	668	582	62	168	27	0	15	82	79	11	102	3	0	4	28	0	0	.289	.374	.412	.787	*D/3H
8	Kendrys Morales	2013	30	SEA	AL	156	657	602	64	167	34	0	23	80	49	6	114	5	0	1	21	0	0	.277	.336	.449	.785	*D3/H
9	Victor Martinez	2013	34	DET	AL	159	668	605	68	182	36	0	14	83	54	10	62	1	0	8	23	0	2	.301	.355	.430	.785	*D3/H2
10	David Ortiz	2013	37	BOS	AL	137	600	518	84	160	38	2	30	103	76	27	88	1	0	5	21	4	0	.309	.395	.564	.959	*D/3H
11	Adam Dunn	2013	33	CHW	AL	149	607	525	60	115	15	0	34	86	76	7	189	3	0	3	2	1	1	.219	.320	.442	.762	D3/H7