IMHO, in Arizona in that engine I'd run a 10W-40 or 15W-40, Full Synthetic oil.
The question about adding extra zinc is a big one, I'm not an expert, so maybe read the link below, however by going to a heavier weight synthetic oil
you'll get more protection from that and the newer chemistry of modern synthetics. If the FJ40 still has a cat that's something to consider also in relation
to extra ZDDP, ZDTP, etc
Almost everyone who’s into cars has an older vehicle that they love. Whether it’s the feeling that they get behind the wheel or the way the car looks parked in their driveway, they pour their time and effort into maintaining the car and keeping it looking like it’s just been driven off of the...
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If you want to know how your engine is wearing with the current oil you could send in a used engine oil sample to Blackstone Laboratories.
Only thing is if you don't put many miles on the engine between oil changes you may not have enough time/miles on the oil to
see meaningful results ie: you'll want something like 3000-5000 miles on the used oil IME (unless something major is happening
to the engine then the problem may show up sooner in the oil).
I've used Mobil 1 Synthetic oils for decades in my LC's but recently switched to
Chevron XSP 400 15W-40 Full Synthetic which is approved for both Diesel and Gasoline engines. No issues.
As important is which oil filter you use, never use the generic white box or other cheapo oil filter most Quick Lube shops install. Spend the money
on the more expensive filters designed for one year and 15,000-20,000 miles IMO.
