The volvo owner manual states that the last engine with a O2 maintenence schedule was the B23. Supposedly the B230 has a lifetime O2 sensor, however these bricks last a long time, so you may want to change it. It is not that hard to swap out the old sensor with a new, just modify a wrench to fit. I have 172K on an original and still get normal gas mileage. 18mpg in the winter is not bad for 240's, the car runs richer longer because it takes longer to warm up so you use more gas without getting more miles.

The T-belt and valves should not affect MPG, unless the valves are seriously out of whack. I would do the T-belt soon if you don't know how old it is so you don't get stranded. When I did the one on my 240 there were hundreds of cracks in the outer shell of the belt, I seriously do not know how it still stayed on and it must have been 100K old, just my guess at the age. This one did last a while, but you never know...FYI the are supposed to be changed every 50K.

The valves do not need to be adjusted very often on these cars as the clearances do not need to be as tight as some cars so there is plenty of slack to play with in the valves. I wouldn't get them adjusted unless they are making a clicking noise after warm up. Even so, it shouldn't run you more than $100 at a shop if you pay to have it done.

Chuck
--
'88 244 171K, '87 BMW 325e 172K (used to feed a '84 245 'till 227K)