Safe @ 10K w Gulf Coast Filter?

[TDI Jack]

Two Questions Re: Oil Changes:
(1) Am I safe at 10K with my Gulf Coast Filter + Delvac oil? I have faithfully changed the TP element + added 1 quart every 2.5K miles. 80+ percent of my driving is highway and the car now has 77K on it. I do not do oil analysis but am considering doing a full oil + main filter change. Any advise on a cheap but very accurate oil analysis provider would be appreciated.
(2) If I don't need to do a "full" change, how far do you think I can SAFELY go? The Gulf Coast guys say I NEVER have to change my oil if I keep up with the TP changes. And they say dino oil is fine. One post I read on this website had a guy go 50K miles. Your thoughts please. Thank-you!

 

[Lug_Nut]

My trip is usually an hour or more, sufficient to evaporate any moisture condensation that might occur. "Highway" use alone isn't enough. The time at operating temperature is also important.
My decision to use 50,000 miles (actually 60,000) is based on the reduction in the TBN in the synthetic, but non-diesel, motor oil I was using. The oil was already low in TBN and the tp changes and oil top off wasn't enough to keep the level up to where I felt comfortable. I was also stretching the 2,500 tp replacement interval recommendation to 5,000 miles.
I could change at 2,500 and with a newly found local source of 0w-40, VW 505.00 approved, synthetic oil, I could probably completely eliminate draining the crankcase. But at 60k I need to remove the belly pan anyway to change the timing belt. The oil change isn't much more time added. The oil cost savings of a 5k versus 2.5k tp change interval is more than the drain and refill cost at 60k.
The oil analysis I had done would not be cost benefitial if they were continued. Each test was about $30. I had them done at 10k intervals while I was calculating my drain interval. I have only just now amortised the cost of the tests which I stopped 120k miles ago. Call local oil distributors or truck fleets to ask who they use for analysis.
You'll need to decide for yourself if the cost and time of 2.5k intervals is worth the cost of analysis.