Just run 15w40. The clearances in the bearings were made for it. Running synthetic 10W40 might be a little too thin. Synthetic oils hold up against heat breakdown better than regular crude oils, but 15W40 is as low as I would go in a 7M or or 1J/2J engine family. I run 15W40 in my Supra (GTE) and My Cressida (GE) engines. The bearing tolerances in these engines don't call for your typical 10W30 and 5W30 oils. They do need higher viscosity numbers when compared to most other engines.
I don't see any problems with collecting the oil out of a glass jar that has been cleaned with denatured alcohol and dry. You're only looking for mineral deposits coming from the bearing material in the oil in this case anyways. Glycol in case of a BHG, but my gasket has 8,000 on it, so that's not a serious problem.
I used to do lab testing of ground water samples for the city of Denver. We would go to potentially contaminated sites to pull ground water samples from water mains that ran by uprooted gas station underground tanks to search for TEH levels and other petroleum contaminations. I'm very familiar with what to look for in a potential contamination based lab test.
