Oil Questions on 5M

My MX73 didn't come with the owner's manual, likely long gone. I have been able to find the Chilton's book, but it is near-useless, seeing as two-thirds of the information cover two different vehicles.

My questions: is 10-40 too heavy an oil for an Illinois car? At 100k and 28 years, should I stick with the factory oil change recommendations? Did they ship with synthetic or conventional oil? Is it safe to run detergent synthetic if it shipped with conventional? It's neither a SBC or a G54B, so I'm a little out of my element.

lol anything with a filter should have detergent oil. unfiltered oil wants the dirt to settle to the bottom of the pan to keep it from circulating while filtered wants to keep the dirt in suspension so it gets trapped in the filter.

under 30f: 5W30
all temps: 10W30
over 80f: 10W40

Then again I run Mobil1 full synth. 5W20 in my 22RE (just as old Toyota 4 banger) with a quarter million miles. Just depends on the condition of the engine.

Synthetic vs conventional: shipped with SG (I think) conventional. anything newer will work. Synthetic is fine but keep in mind all your soft seals are close to 30 years old so the dirt deposits might be all that's holding the oil in. synthetic cleans those deposits which is like pulling the plug after your bath. new gaskets and seals and you're good to go though.

OK, thanks.