Promised I’d add a bit once I switched from 180F thermostats (Gates) to 190F thermostats (Waxstat). For reference, I live in a mild climate (Seattle) where it’s mostly 40Fs to 70Fs year round. We will see 80Fs during summer, and a heat wave here is over 90F. (I’ve only seen 100F one day since living here the last 15 years) I also have a 1988 HE V12 USA specs.
Here are photos of before/after gauges, both in Park at 850RPM. Forgive the gauge photos, it was hard to get a perfect center without perspective, but trust me, both gauges were right on 180F or 190F. Note my oil pressure is around 42psi at 180F, and around 35psi at 190F. I am currently running 10/40 synthetic oil. (I will soon switch to 15/50 synthetic next oil change.) Both of these are after a 30 minute drive in town.
It’s only been two days, but I prefer the 190F. Background - my XJ-S idles a wee bit rough when cold and as warming up, but eventually gets to a smooth idle.
With the 180F thermostats, it would take 10 minutes to get from cold to 180F, and then about 10 minutes running at 180F until the engine finally settled down into a smooth idle.
With the 190F thermostats, it takes about 10 minutes to get from cold to 190F, but once at 190F, the car seems to settle down into the exact same smooth idle as before.
So really, I’ve just bought myself 10 minutes more of smoother idle, but for me it’s worth it because many of my drives are only 30 minutes.
If I lived in Florida, Arizona, Texas or Australia, I’d probably keep the 180F thermostats as I’ll bet the engine has no problem getting to a hot operating temperature fast
The next tests I will do will have to wait for summer. Before the temps had to go from 180F to 200F before the aux fan kicked on, so I hardly ever saw it on. My guess is it will be much easier to get to 200F this summer.