1999 volvo S70. If I drain 4 quarts of regular transmission fluid can I replace it with 4 quarts of synthetic oil? Secondly, is their any advantage to doing this. Thanks.

 

personally, I don't see the advantage of a 1/3 flush, plus you need to make sure your choice of ATF meets the spec of the car. If you really want to move over to synthetic ATF, you can do a full flush by disconnecting the transmission cooler lines at the radiator with one line going to a bucket for old ATF and a second for pumping in the new. If you feel like doing this as a DIY project, I'd start with RSPIs Youtube vid:

 

Yes, you can mix reg and syn assuming that both are compatible with your particular trans. Make sure, otherwise fatal problems can occur. As for the question of what good it does to do a drain and fill where all the fluid isn't exchanged, the old fluid had additives in it that are now depleted and exchanging some of the fluid is better than doing nothing. In fact some manufacturers recommend only doing a drain and fill vs a flush. I personally would never do a flush using a machine that back flushes or applies more pressure than the system would otherwise produce. I also would never flush using a machine if the trans had not been regularly serviced since brand new as once a varnish has built up it can be very detrimental to loosen up all of that varnish and release it throughout the trans.