87 ran richer and colder, so carbonized.
Get it hotter with a 92 degree thermostat, and do cheapo oil changes every 2-3 weeks for the summer.
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Stef -- 1981 245 B21A SU M46 323000km my volvo pages

I am guessing that the '87 was used by the previous owners mostly for short-trip driving. Even if the oil was changed at 3,000 mile intervals, that only works out to twice a year (figure about 6K miles a year based on current mileage and age). Also consider that the air filter, if changed according to schedule, has only been replaced three times in that car's life - or an average of about six years of service for each filter!

The '86 is also a relatively low mileage car, but I bet that it was maintained better - perhaps the PO's replaced things such as filters and oil based on time, instead of mileage, since the last service.

I'd say the 87 wasn't cared for as well during its life: less frequent oil changes, lousy filters, harder driving between oil changes, maybe low quality oil, maybe some overheating?? Just speculation on my part.
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Thanks everyone for all the help, Doug C. 81 242 Brick Off Blocks, turbo bars and wheels, M46; 86 244, B230, 150k , auto.

All things being equal, IE identical distances driven, similar oil/filters etc I'd say the '87 is a dirtier engine (internally). How do they look when viewing through oil filler cap?

The dirty oil is probably still OK and is doing its job by cleaning and suspending the dirt. Lots of folks claim Auto-Rx works well for cleaning engines but I'd just do more frequent oil changes until the oil starts to clear up.
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Norm Cook Vancouver BC; 1989 745T 204,000KM