1990 740 B23F AW70 LH 2.4 @ 275,000 miles

I have been using Prestone antifreeze, Castrol GTX 10W30, and who-knows-what-brand-weight-or-age in the tranny.

I plan on moving to Mobil 1 in the tranny. I already have a thread on how best to do this, so no further discussion is really needed, save for alternatives for the fluid itself.

On the block I am interested in switching over to Mobil 1 also. No reason. Just because. ;-) If I do this, should I stay with 10W30 or move to something more viscous? The miles have been accumulating and I am wondering when I need to start using something better?

And in the radiator the Prestone seems to be fine. Any reason at all to switch to Zerex G05? My Jeep Wrangler requires HOAT and it works great. It was a hassle to locate it here in town. Now that I have a source is there any benefit to making the switch in the Volvo?

Finally, I remember my manual stating that my engine was designed for 87 octane gas. Today I read a PDF of what I believe to be that same manual, but it calls for numbers that would be Premium grade gas. Which is it?

And YES, I have read the FAQ, thank you. I am asking the specific and the FAQ deals with the general on these topics.
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Mama always told me to eschew obfuscation...