3.4l Venture ticking.

Hi all: My 97 Venture with 120k km has recently developed a ticking which sounds like one of the lifters. Funny thing is the ticking is
absent at start-up and only begins after 10 minutes later. The ticking will last for about 10 minutes and then stop.
So, COLD oil - NO ticking, WARM oil - TICKING, HOT oil - NO ticking During the ticking there seems to be a drop in engine power, I guess a valve not opening. Have always changed the oil like the good book says. Have always used Dino juice.
Any ideas. Thanks
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