just make sure when they weld that the fill/drain plug is pulled so you can vent. i'd hate to hear about someone getting hurt from weld fume build up or some pressure release.
i agree with RR, oil seems to eat away everything it touches over time, especially when it's hot. if you can magnaflux the part, you can be doubly sure there's no pinholes, but that's just the engineer talking in me
as to the standard vs. synthetic, that's a long winded debate with no apparent winner. i'm a dealer for Amsoil, and I only run it in my newer bikes since their sealing capabilities are newer and better. synthetic has a "nice" ability to seep where you didn't think it could, because it does it's job so well, that it can flow out even between gaskets torqued properly. just run a good synthetic blend designed for motorcycles and you'll have the best of both worlds. will you run an oil cooler bypass?