Adding some to conventional oil is fine. Mixing the two is o.k. as well. It can/does leak a tad more than dino oil.
However, a caution on using only synthetic oil in some stick applications. My well-broken in Richmond Street 5-speed got progressively harder to shift over time but I hadn't suspected the Red Line syn. oil as the Richmond tech line advised that just about any oil would be fine (dino trans fluid, synthetic, auto trans fluid). Wrong! I drained the pricey Red Line syn. and added a 50/50 mix of Red Line and dino 85W-90W stick fluid. Within the next few drives, the trans shifted noticeably better. Now I run about 25% syn and the balance cheapo dino trans fluid from Autozone!
The material and finish used for the sliders and/or blocker rings makes all the difference. If you have a hard-shifting Tremec, TKO, Top Loader. etc. with syn. oil, change it and see if shifting becomes easier. It will take some driving around before any change is noted, btw.
