Following comments based on 25 years of using and selling synthetic oil.
Some consider my recommendations biased!

Change to synthetic. It would be advisable to use a good engine flush first (best I know of are made by AMSOIL and BG). If you don't, or won't, flush, watch for sudden signs of increased oil consumption after a thousand or two miles (the synthetic will clean out the gunk left by non-synthetic). If you see that increase, change the filter and top up. If you don't flush, plan to change the first batch after 3-4,000 miles, especially if the oil looks quite dirty..

Synthetic oil is in the viscosity range it shows on the label. It will clean out the old gunk, but unless there were already leaks there, there won't be any new ones caused by synthetic. My latest car, an 88 Saab 9000Turbo, was purchased with 87,000 miles on it. The shop dropped AMSOIL in it, no flush, before I bought it. I ran it a year with no problems.

The best oil on the market is AMSOIL Series 2000 0W-30. Next best is AMSOIL 5W-30 or 10W-30. The service life of AMSOIL in a turbo is 3 times the recommended drain interval.
Mobil 1 is excellent--you can use the 0W-30, 5W-30 or 10W-30. I don't recommend extending the drain interval unless you do some oil analysis.

If you have an automatic, by all means plan to change to synthetic ATF. Again, AMSOIL is my first choice, Mobil 1 next.