I switched and developed a leak at the valve cover. I replaced the valve cover gasket and no more leaks. I switched from Mobil 1 to Redline recently. Its better on the seals and a lot more expensive. Adavnced Auto was selling it for $2 off a qt making it about the same price as Mobil 1. I stocked up.
Cory (resident oil expert) recommended a blend of Mobil 1 and Redline due to the cost issue.
I ran Amsoil in my 100 series after I bought it with 116,000 miles. No leaks. Ran great. I drove it until I sold it with 136,000 miles on it, changed the filter evry 6 months and the oil once a year. I had people telling me I was crazy, it would leak, die, burn up, etc, but it never changed anything as far as I could tell.
My dad used synthetic oil in a Mazda pickup since it was almost new. I inherited this truck at about 160Kmiles. Shortly afterwards, I was driving down the interstate and the crankshaft pulley wrung off. This was due to a not-so-smart mechanic damaging the end of the crankshaft while changing the timing belt. So a little frustrated, I had to pull the motor out of the truck to replace the crankshaft on and engine that otherwise ran beautifully. At disassembly, I was amazed to see how great of a condition this engine was in. It was clean with no evidence of wear, sludge or any other kind of buildup. I did replace the rings and bearings only because I had already bought & paid for them. I sold the truck about 2 months ago with 250K miles & it still ran great.
I wouldn’t run anything but synthetic oil in my cruisers.
I bought my 96 80 series at ~190,000 miles, (Jan 1 '04) dont know what PO had been running, but I did one (maybe two) dino oil change then switched to M1 at ~200,000 miles
my dipstick was dropping pretty regularly
I wasnt leaking oil, (I park on concrete driveway) coolant looked good, tailpipe didnt look smokey or smell too bad (although I understand synthetic will not burn but perhaps it was being atomized and blown out the tailpipe as a small particulate doing who knows what to the catalytic convertors)
added oil & hoped things would seal up eventually
changed the filter a couple times, and kept adding oil (engine oil light came on a couple times - by the way checked PCV valve it is good)
did a complete drop and refill with synthetic.... tried 15 w 50... still having to add oil... decided that I couldnt afford to keep putting synthetic in it and went back to dino at about ~220,000 miles
at 230,000 miles now, oil level seems to drop slower with the dino oil, plus I can get a 5 qt jug of dino for about the same price as 1 qt of synthetic.
I am running synthetic in the diff's and xfer case
IMO if you can put synthetics in an not have be constantly topping off, then its worth it. I wanted it to work on my truck, it just didnt.
I've never tried Mobil 1 but i've tried Valvoline max life on my 92 camry and to my suprise it did seal a tiny leak on the valve cover, so it tried this stuff on the LX (did two oil changes with the dino maxlife) then switched to the synthetic 5-30 maxlife. I've noticed an increase in oil pressure. I've recommended it to a friend who has a 99 expedition. He said it helped. He had an oily spark plug in one of his cylinders and after changing to maxlife and new plugs he inspected the "oily cylinder plug" and it was clean. So it might of helped condition the seals.