70K is not too late. I strongly recommend using a good engine flush (AMSOIL, BG) before changing. If you don't, or won't, flush, then watch for any indication of sudden increase in oil consumption. If you see this after a couple of thousand miles, change the oil filter and top off. Otherwise, you might want to drain the first batch of synthetic after 5,000 miles or so.

Synthetic oil will run cooler, reduce internal friction and wear, give you more protection in case of a mechanical problem. You may or may not seen any difference in power or mileage.

There is more difference between brands of synthetics than there is between brands of non-synthetics. I'm biased, since I've been using and selling AMSOIL for over 25 years now. I would only use a synthetic which uses Group IV base stock (PAO) or higher. That eliminates about all except AMSOIL, Mobil 1 and Redline.

Changing to synthetic in the auto tranny, if you haven't already done so, would be a very good move. Will reduce operating temperature by 20-40 degrees F, reduce internal wear, and last longer between drains. Not cheap, until you compare it with the cost of a new transmission.

70K is still very low mileage. Probably no need to flush.

Mostly you'll notice the benefits of synthetic in turbo life.
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http://www.fidalgo.net/~brook4/oilslubesfilters.html

If you want, there's an Amsoil engine flush (drive 1K miles, then change; but you could probably flush the engine with a 1K run of synthetic). General advice is, you can change over with less than 100K on the odometer.
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David (98 S70 T5SE Black, misc mods (mostly lighting), red calipers) (92 940GLE)