158k high mileage oil recommendations ?

[Box7774]

Im probably gunna get crushed for asking an oil question as I am sure it has been discussed 1000 times. I admit from the get go that I am being a bit lazy and just looking for a quick answer.
New to me 2001 Box base with 158k on it. I know all of you guys are using Mobile 1 and previous owner told me I would be due for an oil change soon so I just plan on knocking it out asap. What I forgot to ask the previous owner was what grade Mobile 1 he has been using.
Sooooo I will ask u guys........what grade Mobile 1 should I use for my car w 158k on it and where do u guys usually purchase the oil filter/crush washer from?

Thanks

ps....if I topped off my oil with example: 10/30 M1 and there was 10/40 M1 in the car would it mess anything up? It would be a very small quantity. The reason i want to keep my oil level on the full mark is my IMS bearing seal was removed....so I need that bad boy to get wet.