can you switch between high milage to synthetic
 

I Have Rx300, 2000 with 200k miles. I was at Firestone and technician was telling someone once you start using high millage you can not change to Synthetic, can someone confirm please and what is the reason?

Theory is the seals get swelled up in dino oil and they shrink with synthetic. As seals swell the moving part wears the seal just enough to form oil tight seal. If you then switch the synthetic would shrink the seal and you may be left with leaking seal. If you had switched earlier in life then the wear due to swelled up (or partially swelled up seals) would not be there. Newer models are now rolling off the assembly line with synthetics.

If you dont have any issues then dont shock the engine. Synthetic oils have advantage .. can take higher temperature and can go longer between changes. So as long as you change the dino oil at good interval (shorter than synthetic), you should be just fine.

Salim

The best of both worlds: https://mobiloil.com/en/motor-oils/m...1-high-mileage

Salim's explanation is the conventional wisdom on why people sometimes see oil leaks after switching to synthetic oils. However, this is quite a controversial topic.

I'd stick with dino oil and just change it every 5000 miles.

Everyone has belly button and theory on oil. Actually, general statement, Salim is not the target.
Swelling and shrinking seals that are DESIGNED and MADE for oil is a bit way too common wisdom.
Thing is, synthetic oil is much higher liuquidity factor than conventional oil. It's more thin, less viscous, under the same working conditions, vs conventional oil. So what it will do, is it will find the slightest pore that it can get into, that conventional oil molecules won't fit through, and leak out.
this is why I keep saying - before you consider synthetic oil, is your engine taking oil in any manner? Blow by, leaks, whatever? As if it does so with conventional oil, you are burning money in bonfire with synthetic in that engine.
Say, engine is tight. Like our 3.0 was. TIGHT. Then sure, try semi synthetic and see what happens. Does well - go full synthetic. Our 3.0 was so tight, I did oil changes with Amsoil every about 15 000 miles and oil was barely darker than fresh. I think, I did 17 000 once with filter change in-between. Amsoil warrants its oil even to 25 000 miles, if you change filter half way. And it makes sense.
To OP. Nahh, you can change whichever way you want to, question is - can you? Adding expensive synth to oil taker is not feasible. And, truly, what for? Will you feel adventurous enough to go for 15-20 000 miles on it? There is no benefit in doing it every 5-7 000 miles. Any good conventional oil will serve the purpose.