The 3800 V6 especially 91 and up with the one piece rear main is a very tight engine as far as oil leaks are concerned..
Also the 3800 was one of the lowest friction engines ever produced, all of them were very emissions friendly even without the Cat.. By 94-95 the old L27 was down to the decimal dust as far as emissions was concerned and all of the vans 94-95 easily met the emissions standards...
The 3800 was so clean that it passed emissions without the need of an EGR valve.. after some burnt valves and issues with cracking manifolds due to higher combustion temps GM pur the EGR back on the 3800's from 93 on.. The 88-91 Vin C also had an EGR and were very clean..
Most all modern oil do a good job of cleaning the motor... Synthetic has smaller molecules in a word that can slip by a gasket that Dino may not..
3800's in general are very well sealed and don't tend to leak much at all.. Can't blame a 20 year old valve cover gasket for leaking, same goes for the oil pan..
I have owned nothing but 3800's over the last 20 years, and have run synthetic and dino and switched back and forth.. Never had a single issue with any of them..
My series III in my van burns nothing, doesn't seem to matter if its synthetic or dino nor does the brand seem to matter..
My series II 3800 in my 03 Bonneville burns about a quart in 4000-5000 miles, it always has, currently at 111,000 miles.. It burns oil the same no matter what I use..
My series II in my 95 Bonneville is exactly the same... Burns 3/4 of a quart in 3000 miles, always has... Its always the same no matter what kind of oil I use..
The original 95 L27 in my van say all kinds of oil in its life.. No matter what was used it never burned a drop..
Not being able to go from synthetic, to dino and dino to synthetic is a myth.. I run synthetic in all my lawn and garden equipment as well, never had an issue..
Kev have you thought about running 10-40 until it warms up a bit in your neck of the woods?