Hi Malcolm,
Firstly on the engine oil and filter. I have been using Penrite 20W60 HPR30 since 2007 in my 4.6 litre engine along with the K & N HP2003 oil filter. I have driven my Rover over 200,000 miles (322,000km) since that time, and I can happily say that you will not have a problem using either product.
The significant jolt going into D and R I suspect is due to your engine idle speed being too high. It should idle around 650rpm in park when hot.
I am no expert with automatic transmissions but to me, it sounds like your transmission has too much fluid. The workshop manual states to run the engine at idle speed for a few minutes, selecting each gear position to distribute the fluid throughout the gearbox. Stop the engine, check and if necessary top up the fluid to the dipstick level mark. Your box sounds like it has possibly 1 to 1.5 litres too much fluid.
Ron