There is an issue that I've seen in use of older cars (because I only buy beaters for under two thousand dollars) where a higher-mileage engine keeps oil pressure better with thicker oil than the manufacture's (for me, typically GM) rating. To me, that's a difference between a rebuild at 175k and at 250k, which is worth it for me. Specifically, I have a 1995 Chevy C1500 with a TBI 4.3 that recommends 5w-30, but at that weight it burns more than a quart of oil at 3000 miles and hot oil pressure is around 30 psi at idle (45 under load) whereas with a 10w-40, it maintains 40 at idle and 60 under load, which I think of as better for preventing engine wear. It also burns significantly less oil over time (1 quart over 6000 miles or so). Of course usage variables influence oil loss and such, but bang for buck, using a heavier oil on high mileage engines has served me well, especially on older American engines.
With Japanese engines, I understand that tolerances tend to be a bit tighter, and oiling systems tend to wear less over time, and maintaining the manufacturer's cold temp rating has merit for ensuring lubrication on startup. I also haven't seen oil pressure issues with maintaining manufacturer recommended oil rating on Toyotas specifically, though I tend to err on the side of caution on the hot rating with a heavier oil.
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