You'd probably better off computing the stripped-bonds spread, which has ample liquidity in both cases I think. But again, it may not be meaningful right now, with mass stampedes in the money markets.

Wait a few more months to draw conclusions. I don't think Exxon could have a better rate than the US, at least no for very long, because of the following macro-arbitrage argument: if the liability is in USD, exxon will pay it in the US, where its legel entities would be seized/taxed to death if they still had an oil rent there, and the federal government was broke. liabilities denominated in other currencies / from non-US subsidiaries are another matter, but I don't think the pool of exxon bonds has many like these.

Pierre